I think it like this. You are going to post a video on Youtube where expressions and feelings are to be conveyed. Quite pointless if you post a video where you look like a dead fish. Those who post the videos know this so they allow themselves to "pulled on" with emotions. Maybe overplay in some cases but still quite authentic.
If you see it as an art form (not much art in itself but for the principle, for the sake of the argument) then the same principles apply there as to create a good movie. Different quality levels on those videos. Take them (or not) for what they are. A little entertainment for the moment only.
Then there is a big difference between these reaction videos. There are, as suggested in the thread, videos where it is more a matter of analysis of song, composition and so on. For example this: