Reactions do fall into the fair use category because it's a commentary.
Again, no. Even if they are classified as a commentary (a very gray area and debatable itself), reaction videos (more often than not) show the video they're reacting to in their entirety. A huge no-no.
You say that Game Developers authorize people use of their games in lets plays which isn't true, there are plenty of channels (which I have seen from this forum) that get into trouble with copyright and strikes. Not all Game Developers allow for their games to be uploaded or even monetized especially in specific instances such as cutscenes and in some cases: reviews.
I never said all game developers authorize their game footage usage. It is an ever changing group of developers and publishers who allow it and it is up to the YouTube channels to do their research and know in advance what games are and are not allowed.
The channels who don't get into any sort of trouble are the ones who use games that are authorized, but that still remains up to the game developers.
Reaction videos are completely different as the vast majority of videos out there do not give permission, are not part of ContentID, and have no way of claiming the videos' earnings for themselves. The only recourse is a copyright strike which, in the majority of cases, is completely justified.
Furthermore, most people believe they can just record, edit, upload and monetize gaming videos without ever getting permission from the Developers or seeing if they have guidelines which allows for people to upload and monetize. That's just like guys like Jinx. In addition, Game Developers can monetize other people's gaming videos if they wanted (and it happens) just with how Reaction/Commentary type channels have their videos monetized by other Creators. So yeah, the situation in regards to fair use, reaction and gaming videos are really similar. All that mess I've mentioned can be a huge gray area here on YouTube.
Interesting how you said they have the right to post reaction videos (as well as gaming videos), but quickly change your tune after my response and give reasons on why people can't.
In the end, gaming videos and reaction videos are completely different... especially since, with a quick Google search, one can see exactly which game companies allow footage and monetization and which ones don't. That simply isn't an option with creating reaction videos and, as I've stated, the vast majority of reaction videos I've seen would quickly fail the four statutory factors of Fair Use.