Why does Meta allow posts that are not questions?

There's a reputation limit, so users can only get 200 reputation per day (some exceptions apply). The quoted reputation gained for these questions and answers will not be accurate.

Protection doesn't happen immediately, if something was protected by Community it means that a number of answers have already been posted (29 in this case). Low reputation users could have got the reputation if they'd got in quickly enough.

Reputation on Meta is slightly meaingless anyways... it's here just because this isn't technically a meta site. It's imaginary internet points on a site about the interaction with sites where you gain imaginery internet points. If you participate regularly the reputation will come, whether or not you manage to get the one highly upvoted "Congratulations!!!" answer.

Whether Meta should be used for announcements is quite a contentious issue, especially announcements that are percieved to be social. Generally "questions" about new staff members are left alone, but announcements of new products are often closed multiple times. Meta Stack Overflow is perhaps even harsher.

Whatever your personal opinion, Stack Overflow (the company) has decided to use their platform for announcements. This means that the meta sites throughout the network (but especially Meta Stack Exchange) are used for announcements.

Until Stack Overflow changes their mind announcements will continue to be posted and no extant answer by a regular user will change this fact. The answer should maybe be edited to reflect that announcements might be posted.

It still suggests that Meta is supposed to be used for asking questions, not advertising new features or announcing new community managers.

The question references the following answer How do I participate in Meta and not die trying?

That answer explicitly states:

Meta Stack Exchange is a special place for suggestions, bugs, support questions, and discussion about the Stack Exchange sites.

Suggestions, bug reports and discussion are not (explicit) questions. So clearly the site is not limited to only asking questions.

That makes "advertising new features or announcing new community managers." on topic.

A tool is whatever you need it to be. My dad used to hammer in nails with a pestle for years until we got multiple hammers and insisted he use it.

While traditionally, announcements were made over the blog, it's a bit of a clunky tool at the current point, and to a big extent is more aimed at an external audience than the core user base.

On the other hand, there's this old, almost forgotten diagram:

...which once resided in the tour page - and that really does reflect meta well - least for many of us, it acts as a community built repository of knowledge (aka a wiki). It's a place for announcements, and acts almost as a secondary blog. While I'd bristle at calling it a forum in the modern sense - it is a place for discussion. Not sure about these Digg/Reddit things.

In a sense, folks at SE found that the 20-lb sledgehammer worked less well for their needs here than one of these:

_{By Agolz12 - Own work, CC BY-SA 4.0}

It's a multi-purpose tool that does many things. It might not be the best tool for driving in stakes but... that's not what we always need.

So meta is used by Stack Overflow in this way since it's the most efficient way to get information through to the broader Stack Exchange Userbase.

Why Stack Overflow is allowed to post that kind of questions is explained in the help center and the FAQ link found by @Sonic, and in @ben's answer. Let me focus on

Furthermore, and perhaps of most importance, why do we only let these free points for people saying things like "congratulations" go to people that already have 10 points reputation (see image below)?

On that particular post, there are a few answers which have been deleted:

Just being able to post the umpteenth variation of 'Congratulations' is not a guarantee for free reputation. In general, on these kind of questions, you have to be quick (the 'fastest gun in the west'), which means you have to be either very lucky or to be here all the time. Guess what, the same users who are here all the time are the ones which already have a lot of reputation. This holds true for every Stack Exchange site in the network, including Meta.