Some people view Let's Plays before buying a game. People aren't going to go to the trouble of recording Let's Plays without getting any kind of ad revenue from it. So people aren't going to bother recording Nintendo games anymore.
And they can just delete all the Nintendo videos they already have up. So now Nintendo doesn't have anyone promoting their games (for free to Nintendo) and isn't getting any of the very small amounts of ad revenue and is competing against a bunch of other games that do have Let's Plays up online.