Effects from spellcasting are balanced around the action economy as-is, and the suggested house rule allows many spells to be cast additionally in a single turn with no restriction, significantly changing the balance from RAW. For example, the way that cantrip damage scales is inline with damage expectations from other classes without using resources, whether Fighter's extra attack, Rogue's sneak attack, Monk's damage scaling etc*. Allowing the damage output to double for these "basic" attacks, but only for spellcasters significantly improves them when comparing between classes.
In addition, most pure spellcasters have limited alternative uses of bonus actions. That means even if they only use the bonus action to cast their most effective cantrip per turn, the impact is a strict power-up.
Players could also build around the house rule, boosting cantrip damage or effectiveness (some feats, Warlock invocations and cleric domains improve some cantrips), and eschewing spells that would otherwise require using the bonus action, so they are not exposed to opportunity costs. The Cleric would never cast Spiritual Weapon, using a 2nd-level slot to get maybe +1d8+5 damage per round, limited by foes being within 20' of last attack, when they could get +3d8 damage attack per round using a bonus action Sacred Flame to hit anything in range.
If playing a high-powered game, additional house rules or homebrew items could address the balance between player characters. That might be the case in the example you found. Or they might simply prefer more powerful spellcasters.
NPC enemy spellcasters will presumably also get access to this, making them more boss-like (in that they get to perform extra actions on their turn). If the DM is setting up encounters with this in mind, it could be less of an issue, and balancing between PCs and NPC enemies is a bit looser in any case.
In general, effects that alter the action economy - whether by adding/removing actions on a turn, or changing an action type - can be very strong and can have a lot of consequences for balance. That doesn't prevent them being in the game, but you will usually see them carefully limited and/or tied to a limited resource.
* These example classes all have options to increase damage output using bonus actions, but it comes at a cost. For the Fighter, a loss of AC, for the Rogue opportunity cost for all the other things they can do with Bonus action, for the Monk they spend Ki which is limited.