Your rule would be the right way to do all this. You could even have a mechanical royalty that applies by default where you can train on anything that hasn’t set rules and a preset rate.
"It's perfectly OK for a police officer to observe a street corner, see crime happening, and go take action, therefore, building a complete, panopticon surveillance system that watches all street corners simultaneously and deploys police to take action, is perfectly OK, too, since that is exactly the same thing."
In return, I'll share my own reasoning:
Human Life is finite, and there is a real opportunity cost to learning (say) to copy Picasso's style -- you could've been doing something else in that same Time.
However, if you're training a model on the entire career output of dozens of artists, it only needs more electricity + GPUs to do this.
And, transposing this back to the Human realm, there's no way one person can put in that kind of wide effort to learn how to copy dozens of artists' styles in one lifetime.
I really like the way you put it too -- it's a bit more succinct.