The core market for the feature is schools, and that group is generally okay with the filter being conservative and low-granularity (read: little thought required for configuration)
Users: Somewhere in this nation, there is a high school administrator who consents to students learning about politics. There might even be more than one.
Another company could pick up the slack. Hell, PragerU could do it. Nothing stopping them from brokering their own ad deals to run prerolls on video they host with their own infrastructure. A few schools might allow access, and many will just black-hole prageru.com in their firewalls.
At Google scale, one has to weigh the risk of false-positive and the risk of false-negative on a low-granularity feature like this, and the risk of error due to misconfiguration being blamed upon YouTube if it's turned into a high-granularity feature. Because the failure mode for YouTube if they false-negative something that is visible to students who should not have seen it (and generate a negative press cycle for themselves) is that schools choose to black-hole youtube.com instead of bothering with Restricted Mode at all. That's what was happening before they added the feature, and it's the reason Restricted Mode exists.
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...
https://www.youtube.com/watch?v=KZWasc1rC_U
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).
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.
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?