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).
