They do have power over it. They can force the creators to make sponsor-free versions for premium users. It can simply be part of their T & Cs just like they don't allow porn etc.
Don't forget the creators already get a lot more money for a premium view than an adsponsored one. And nothing for an adblocked view.
Besides, I deal with YouTube. Not the creators.
While I agree overall, for me it's not as clear cut with sponsors as I do genuinely find some sponsored content very interesting and informative.
A creator might not have the means to buy the equipment, so not being sponsored would mean not making that content, which would be a net loss in these cases in my view.
That said, sponsored segments for BetterHelp, NordVPN and similar can f right off.