IE why isn't it that an artist could say, hey I'm letting you see this painting, but you are not allowed to sit down with a canvas and learn how to reproduce it? Because you can do that in galleries - no photos, no reproductions.
So actually building a machine there, under the cover of darkness, that learns from your work so you can produce new work, why is that allowed in the first place? Certainly wouldn't be at a museum.
The key thing here is - if you want artists' data, you should ask for it. They didn't. This would be equivalent of training a Github CoPilot on every available piece of code in existence, ever, instead of what they had available. Why should that be allowed? So if I built some toy code in 1996, and happened to post it on usenet, and it's a great implementation of X, why the heck is CoPilot allowed to read it? It's my property.