The problem is what is an appropriate format of "criticism"?
Like, if you use select bits and pieces of a video as examples of points you want to criticize, that's one thing...but if you're playing a large portion of a video (and just commenting on top of it), that's another things entirely. Like, Mystery Science Theatre 3000 is *not* fair use. They had to pay for licenses to use the movies they commented on. RiffTrax gets around this by only doing the audio commentary -- and viewers have to separately have the video and then sync up the audio commentary to the video.
These reaction videos where people just play others' youtube videos while they insult them or whatever -- it's definitely ethically and legally spotty -- it's just that no one has the time/money/energy to file lawsuits.