> but is training an AI copying?
If the AI produces chunks of training set nearly verbatim when prompted, it looks like copying.
> And if so, why isn't someone learning from said work not considered copying in their brain?
Well, their brain, while learning, is not someone's published work product, for one thing. This should be obvious.
But their brain can violate copyright by producing work as the output of that learning, and be guilty of plagiarism, etc. If I memorise a passage of your copyrighted book when I am a child, and then write it in my book when I am an adult, I've infringed.
The fact that most jurisdictions don't consider the work of an AI to be copyrightable does not mean it cannot ever be infringing.