I'm intrigued - can you point me to any credible commentator or article that makes the argument "it's illegal to train on copyrighted material without a license to it" - that seems entirely contrary to the spirt (as I understand it) to copyright law, which is it grants you a right to copy.

I like that you bring up music - almost every musician you have ever listened to (some exceptions of course) developed their talent by learning from others - chords, bridges, etc... And I'm just as certain that close to 0% of them had a "license to learn" from their material.