Based on the language from Valve, it sounds more like legal protection for themselves than a judgment from an ethical perspective.

Your question isn't a bad one, but the battleground over copyright ownership probably isn't one they're weighing in on here.

I think I'm starting to understand... If I go to an art gallery that allows photos, take some photos, and share them with a friend who is learning to be an artist, that seems to be generally ok and does not feel unethical. But if I take those photos to an underground sweatshop and use it to train a thousand people who are mass producing art for corporate use, that seems wrong.

If I think of the AI as a human analog, then I have trouble seeing the problem with it learning from the same resources as humans, but if I see it as a factory then I see the problem.

If a human artist learned by copying paintings, they still create original work. An AI simply copies.

I mean, do they publish games with art made by humans who learned from works the human artists did not own?

You know plagiarism is a word right? Artists/Writers still strive to have a style unique to themselves....