US fair use considerations:
the fair use of a copyrighted work for purposes such as criticism or comment (subject to):
Reviews and Let's Plays absolutely have merit as COMMENTARY and CRITICISM
the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
OK it's commercial, but the commercial value is from the COMMENTARY and CRITICISM, not the mere reproduction of game footage
the nature of the copyrighted work;
the amount and substantiality of the portion used in relation to the copyrighted work as a whole
Footage, even a LOT of footage, doesn't amount to substantial reproduction of the copyrighted work. Because gaming is an interactive medium and you're only seeing one playthrough, and even a thousand playthroughs wouldn't actually be the game. And also such showing is necessary for the COMMENTARY and CRITICISM to take place.
the effect of the use upon the potential market for or value of the copyrighted work.
Either zero (because it's not a reproduction), or in fact net positive (the exposure). The reproduction of game footage is no substitute for purchase and certainly does not devalue it.
The COMMENTARY and CRITICISM may well devalue the product, but that's not the part that we're considering for fair use.
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Conclusion: All of these Youtube videos are (probably) fair use even with the monetisation. The companies shouldn't have the final say on whether a video stays up.
