Other commenters (https://medium.com/@ephromjosine/okay-prageru-lets-look-at-w...) note it may simply fall under "mature" for dealing with topics of violence, war, and terrorism.

From that link:

Mature subjects: Videos that cover specific details about events related to terrorism, war, crime and political conflicts that resulted in death or serious injury, even if no graphic imagery is shown.

. . . .

You can’t post videos on Terrorism, War, Crime, and Politics.

It's interesting that all USA politics is acknowledged to be "political conflicts that resulted in death or serious injury", but somehow I doubt that MSNBC will be held to this standard...

They appear to be. Only a handful of PragerU's videos are marked restricted. A cursory check on https://www.youtube.com/user/msnbcleanforward/videos with restricted mode on and off shows a handful of their videos are also marked restricted, for example:

https://www.youtube.com/watch?v=KZWasc1rC_U

https://www.youtube.com/watch?v=TSFQos3XTEQ

https://www.youtube.com/watch?v=2ZWtdFVU904

Mentioning coronavirus is violent or demeaning? I am more and more confused...

I have no idea by what criteria, precisely, these videos or the PragerU videos were marked restricted. Neither does PragerU, for that matter; Google doesn't surface that level of detail on the Restricted algorithm to end-users. I've been making educated guesses at the detailed cause, but it's somewhat irrelevant; the answer is, really, "Google's algorithm deems this content should be restricted."

By that criteria, MSNBC and PragerU appear to be treated the same way.

(Edit: FWIW, I went ahead and clicked through one video and that micro-sample should probably be flagged for "politics;" pundit accusing the President of, at best, being ignorant of science, and at worst, misleading the American people intentionally. That's clearly inflammatory political content).

That's key to the current frustration with YouTube, outside of constitutional grounds: as a platform, it's become wildly unstable to depend on, as you simply are not given the ability to understand where it will or will not draw the line, and their application of their standards leaves such an ambiguous and inconsistent trail of evidence that it's harmful to everyone who tries to rely on it.

I know you keep linking this blog post as if it proves a point, but its argument is tenuous at best.

The crux of the author's case is that PragerU's videos fall into the category of "videos on Terrorism, War, Crime, and Politics."

This is a remarkably wide net to cast. Surely you can think of countless YouTube channels which were not subject to the same scrutiny that Prager was, yet obviously have videos on politics and war.

At the end of the day, Google disagrees with Prager's politics. I don't think it's much more complicated than that.

Under the same Restricted Content scheme, Google also flagged LGBTQ+ videos (https://www.theverge.com/2018/6/4/17424472/youtube-lgbt-demo...). I don't think anyone argues seriously that the company's politics (in general) are at odds with supporting LGBTQ+ causes and people.

It may be worthwhile to ask not what YouTube wants, but what the audience for these features wants. Are PragerU videos restricted because most schools think they're worthy of restriction?