Whether it’s legal slash fair use to train on copyrighted material is only one of the questions currently being asked though. There’s a separate issue at play where these companies are pirating the material for the training process.
By comparison, someone here brought up that it might be transformative fair use to write a play heavily based on Blood Meridian, but you still need to buy a copy of the book. It would still be infringement to pirate the e-book for your writing process, even if the end result was legal.
If they would buy material at a large scale, the seller might require them to sign a contract that requires royalty if the material is used for training an AI. So buying legally is a way to put yourself into a trap.
The only thing I've been able to find is the note that since copyright is federal law, state contract law actually can't supersede it, to wit: if you try to put a clause in the contract that says the contract is void if I use your work to make transformative fair-use works (or I owe you a fee), that clause is functionally unenforceable (for the same reason that I don't owe you a fee if I make transformative fair-use works of your creations in general).
By comparison, someone here brought up that it might be transformative fair use to write a play heavily based on Blood Meridian, but you still need to buy a copy of the book. It would still be infringement to pirate the e-book for your writing process, even if the end result was legal.