Why are the Dementors loyal to anyone other than themselves?

Question

During the first five Harry Potter books the Dementors were (notionally, at least) loyal to the Ministry of Magic. There's the odd occasion when they indulge their carnal desire to suck out the souls of innocents, like at the Quidditch match in Prisoner of Azkaban. But, by and large, they are obedient to their human masters.

In the last two books the Dementors switch their allegiance to Lord Voldemort. Dumbledore explains that the reason for this change in loyalties is that the Death Eaters can offer the Dementors much greater freedom to feast on human misery.

"Voldemort has returned," Dumbledore repeated. "If you accept that fact straight away, Fudge, and take the necessary measures, we may still be able to save the situation. The first and most essential step is to remove Azkaban from the control of the Dementors -"
"Preposterous!" shouted Fudge again. "Remove the Dementors! I'd be kicked out of office for suggesting it! Half of us only feel safe in our beds at night because we know the Dementors are standing guard in Azkaban!"
"The rest of us sleep less soundly in our beds, Cornelius, knowing that you have put Lord Voldemort's most dangerous supporters in the care of creatures who will join him the moment he asks them!" said Dumbledore. "They will not remain loyal to you, Fudge! Voldemort can offer them much more scope for their powers and their pleasures than you can! With the Dementors behind him, and his old supporters returned to him, you will be hard pressed to stop him regaining the sort of power he had thirteen years ago!"
_{(Goblet of Fire, Chapter 36, The Parting of the Ways).}

Yet wouldn't the Dementors have greater scope for their powers and pleasures by operating as independent agents rather than as servants of Voldemort? They may have had greater independence under Voldemort to patrol Muggle areas freely, to breed relentlessly and to use the Dementors Kiss on innocents - things which they weren't allowed to do under the Ministry. But they were still slaves who had to obey the commands of humans. They were still used as the dog's bodies who guard the prisoners (albeit they were imprisoning Muggleborns rather than real criminals). They were still limited in who they could attack (since Voldemort greatly values wizarding blood and wouldn't permit the Dementors to carry out the Kiss on the majority of law-abiding wizards who weren't opposing him). They were still bossed around by people like Umbridge. They were still slaves.

What exactly did the Dementors get in return for serving Voldemort? Wouldn't they have been better off just acting as free agents, attacking whoever they wanted to and obeying nobody? Since all they seemed to really want in life was to spread despair and suck out the odd soul it seems to me that they could've fulfilled this desire more frequently if they stopped serving others and began serving themselves.

Why exactly were the Dementors motivated to serve either the Ministry or the Death Eaters, given that all of their human masters restricted their activities in one form or another?

Answer 1

They may have benefited from their victims being together.

Both the Ministry of Magic and the Dark Lord stationed the Dementors at Azkaban, where the prisoners were all held captive and grouped together in one building. They couldn’t use magic, and they were unable to escape the Dementors or fight back. The atmosphere of Azkaban was beneficial to the Dementors, because just being there weakened anyone who was.

“Forget it, Harry,’ said George bracingly. ‘Dad had to go out to Azkaban one time, remember, Fred? And he said it was the worst place he’d ever been. He came back all weak and shaking … They suck the happiness out of a place, Dementors. Most of the prisoners go mad in there.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 6 (Talons and Tea Leaves)}}

This was likely preferable to hunting lone victims, who would be able to run and use magic against them. Their supposed “loyalty” to the Ministry, then the Dark Lord, meant they were guaranteed a large amount of victims who’d be unable to fight back in any sufficient capacity. Dementors thrive in places like Azkaban, filled with decay and despair.

“Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope and happiness out of the air around them.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)}}

Under the Dark Lord’s rule, they were able to congregate in the courts where Muggleborns were put on trial, as well as anywhere else the Death Eaters wanted them to spread despair.

“Dementors, he thought.

And as he reached the foot of the stairs and turned to his right, he saw a dreadful scene. The dark passage outside the courtrooms was packed with tall, black hooded figures, their faces completely hidden, their ragged breathing the only sound in the place.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 13 (The Muggle-Born Registration Commission)}}

The Muggle-borns were likely more plentiful than the criminals, and the Death Eaters were more willing to let them actually suck out souls from the uncooperative. In addition, once the Death Eaters were freed, and the Dark Lord had control over the Ministry, they may have been also allowed to retake their posts at Azkaban, but guarding the Death Eaters’ prisoners instead, as well as having the Muggle-borns to feed on in the courts. Harry, at least, thought they were back to guarding Azkaban as well (though Luna was actually held at Malfoy Manor).

“It was nearly dawn when he remembered Luna, alone in a cell in Azkaban, surrounded by Dementors, and he suddenly felt ashamed of himself. He had forgotten all about her in his feverish contemplation of the Hallows. If only they could rescue her, but Dementors in those numbers would be virtually unassailable.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 22 (The Deathly Hallows)}}

Without human “allies”, they’d be unable to have a specific place to infest. Not giving “allegiance” to anyone would give them more freedom, but likely less to feed on as well. Their priority was having the highest possible number of victims to feed on - their “loyalty” was so they’d be provided victims.

“Think that matters to them? They don’ care. Long as they’ve got a couple o’ hundred humans stuck there with ’em, so they can leech all the happiness out of ’em, they don’ give a damn who’s guilty an’ who’s not.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 11 (The Firebolt)}}

Only by serving as guards would they be able to get hundreds of victims in one place.

Wizards also may have had the power to expel them.

After Kingsley Shacklebolt took over as Minister of Magic, the Dementors were removed entirely from Azkaban, meaning wizards would have the power to do so.

Under Kingsley Shacklebolt, Azkaban was purged of Dementors. While it remains in use as a prison, the guards are now Aurors, who are regularly rotated from the mainland.
_{^{- Azkaban (Pottermore)}}

It’s possible they saw cooperation with humans where they were provided with victims as better than being removed by whoever was in charge.

Answer 2

They would end up with less victims

The "only" magical power Dementors have is to make people miserable and, in their best-case scenario, to suck their souls. Witches and wizards have the potential power to always fence them off resorting to the Patronus charm, if they know how to use it. It is true that probably most witches and wizards are not familiar with the charm, there are many instances where this is remarked. In Hermione's words:

Harry, I can't believe it. ... You conjured up a Patronus that drove away all those dementors! That's very, very advanced magic. (PA21)

However, it is reasonable to assume that most people in the magical world don't know how to conjure a Patronus simply because it is a difficult charm and there is no need to know it, since they assume that Dementors are kept in check. In other words, the costs of learning the charm outweigh the benefits. If that were to change, most people would adapt and learn it.

Moreover, there are not only the individual witches and wizards. The magical community is embodied in the Ministry of Magic, which detains what the Muggle world calls monopoly of the legitimate use of physical force. No matter who actually detains this power (the Minister or The Dark Lord), there would be a reaction to the Dementors becoming independent. At the very least, Dementors would lose control over Azkaban detainees. In the worst-case scenario (for them) they could be systematically repressed and prevented from making anyone miserable by the Ministry of Magic.

Let us compare the possible regimes and the shares of people Dementors could make miserable:

1. Under the control of the MoM: prisoners of Azkaban, outlaws on the run and other people targeted by the Ministry or its officials (Harry in OP1);
2. Under the control of The Dark Lord: prisoners of Azkaban and outlaws on the run (including Mudbloods and anyone close to the Orden of Phoenix or the DA), Muggles;
3. Independence, in the short-run: everyone not able to defend themselves;
4. Independence, in the medium-run: possibly nobody (before the fall of the Ministry), Mudbloods and Muggles (after the fall).

It is not hard to see that independence would increase the share of potential victims in the short run, but decrease it in the medium-term, even dramatically so.

Dementors have always chosen the optimal regime to maximize their victims in the long-run.

Answer 3

The dementors are essentially hostile actors with limited power. In numbers, they can be a threat to single captive wizards. But if an organized group of wizards decided to come after dementors, there's not much they can do to defend themselves. They have to vassalize under a powerful group who can protect them. If independent dementors roamed the land and preyed on anyone they find, wizards would surely unite to either exterminate or contain them. In the latter case, though the dementors would be allowed to survive (perhaps out of ecological/environmentalist concerns), they would have no or little access to prey.

Neither the Ministry nor Death Eaters would ultimately tolerate aggression from dementors, thus they must pick a side. As is stated, Voldemort can offer them more victims, and they must also believe that Voldemort is more likely to win (especially if they join and subvert Azkaban). One could also reasonably expect that since Voldemort is less humanitarian, his rule would not go out of its way to protect the weakest members of society from dementor abuse (ie. outside Azkaban), since they would view it as a sort of social Darwinism.