Of course people reduce their free contributions to Stackoverflow. Stackoverflow is selling then out with the OpenAI API agreement and countless "AI" hype blog posts.
I don't think it's the main reason. People don't care whether someone is selling stuff they create on a platform. Big social media has been doing it for many years now e.g. Facebook and yet it's still there. You come to SO for answers, why would you care that someone is teaching some LLM on them later?
This doesn't make the slightest bit of sense. The people who would be concerned are the ones who are providing answers. They are not coming to SO solely to get answers.