You have to understand where the ad is coming from. With Premium, you play YouTube to not interject extra ads. They can't really do anything with the content itself.
They are however in power to make a rule that prohibits third-party advertising in videos, delete any videos that break the rule and ban repeat offenders. Every forum back in the day explicitly prohibited any kind of commercial content - I'm actually surprised every social network is allowing all kinds of commercial promotion, even more so considering they are themselves in the business of advertising (thus unofficial ads are actually eating into their bottom-line).
