I'm a bit uneducated when it comes to copyright law. All I know is fair use pertains to being able to use content not owned by you freely mainly for commentary, criticism, and/or parody. So far, I haven't really seen any videos of Nintendo games being blocked under those circumstances. I could be wrong, but I don't scour around YouTube or comments just to search if anyone had those issues. Though personally, as much as I am a fan of certain youtubers, I'm not really invested in their issues with the law and such. I am educated enough as a gamer to know what games interest me or not. I don't necessarily need to see long walkthroughs unless I'm stuck at something (though I just use guides).
YouTube itself is in a grey area because almost anything can be posted (except the more obvious ones). I'm not sure about the policies of NicoNico so I'm interested in seeing the difference between YouTube and NicoNico.
Regardless though, I'm not a fan of pirating. Sure, there are Japanese exclusive games I'm interested in playing, but to pirate games for the heck of it (and to spite Nintendo), I'll leave that to whoever cares about it. If I have FE Blazing Sword or Fates, why should I pirate them? Plus Nintendo hasn't really done anything wrong to me as a consumer, personally. They've given me so much as a gamer anyway. I'll always remember Mario World and ALttP being my first games that I've played. I'll remember playing Mario 64 for the first time. I'll remember seeing Smash 4's final roster and putting it up there among the greatest of all time. So I am not obligated to do such thing. If you want to do to spite Nintendo, fine, it's your life and your choice.