Why? They put up content on the web that people can watch without paying for it. Why would I stop viewing that content just because I don't want to pay their premium price?
Be it from ad or from YouTube premium, Google is still getting roughly the same amount of money out of you. If you dislike Google’s actions so much, the only ways to not give them money are either to stop watching YouTube altogether or use an ad blocker. In fact, watching with ad blockers is still the worse option since you are still increasing the YouTubers’ popularity and making the site more visited by other people (who are probably not using an ad blocker)