Valve may be anticipating that it will become illegal, in which case they would have to pull games from the store and issue refunds. They may be waiting to see which way the wind blows on this before they get their revenue stream mixed up with it.

Do you have a legal decision that AI art forms no derivative of a copyrighted work, vs a derivative of ALL the copyrighted work in its training set?

There is no new law, there is ambiguity with the lack of current case law. There are certainly people who think generative art based on scraping the internet is infringing under current legal standards. We won't know until people go to court and judges make decisions.

Valve's insistence in the face of their own liability for selling derivative works seems a sound business decision here, at least when selling in markets where it is an open legal issue.