The only thing stopping them is the fact that anyone who wants the data can just utilize the federation protocol to take any data they want, and there's not a lot anyone can do about it. You can't sell something that's trivial to get for free.
If the question you're really asking is "what's stopping content on Lemmy/Mastodon/etc from being used to train an LLM?" the answer is, nothing.
mass user exodus to one of the many other identical Instances. Also, data brokers prolly aren't interested in going after each Instance because no one instance has enough data to make it worthwhile. Yet again, the fediverse proves its resistance to enshitification.