YouTube's copyright policy defaults ad revenue to Nintendo, and not content creators.
Nintendo started to issue ID Claims on various videos on YouTube that include footage from their games, including popular "Let's Play" videos, as well as produced game coverage.
Since YouTube does not have a system in place for dual revenue sharing, companies that claim copyright can collect the ad revenue made from the video in question, instead of the content creator. This is different from copyright claims where Nintendo could just delist videos containing video from their games.
"As part of our on-going push to ensure Nintendo content is shared across social media channels in an appropriate and safe way, we became a YouTube partner and as such in February 2013 we registered our copyright content in the YouTube database." says a Nintendo representative, speaking to GameFront. "For most fan videos this will not result in any changes, however, for those videos featuring Nintendo-owned content, such as images or audio of a certain length, adverts will now appear at the beginning, next to or at the end of the clips. We continually want our fans to enjoy sharing Nintendo content on YouTube, and that is why, unlike other entertainment companies, we have chosen not to block people using our intellectual property."
Nintendo has yet to comment on the situation involving ad revenue no longer going to the content creators.