Fair use, that's the one, not free use like I said. That's something elseMaybe in the US you could argue that under fair use but I doubt you'd get very far. Productive commentary would be more of a review format using clips not just playing the whole thing going "OMG!" like many react videos do (not all).
A quick look online shows plenty of them are taken down or demonetised, guess some get away with it.
Yeah it's hardly a "free lunch" but plenty of people do it just to grab views which I'm sure can increase their popularity for other monetised content.