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.