Cassette Tapes and Private Copying Levy.
https://en.wikipedia.org/wiki/Private_copying_levy
Governments didn't ban tapes but taxed them and fed the proceeds back into the royalty system. An equivalent for books might be an LLM tax funding a negative tax rate for sold books e.g. earn $5 and the gov tops it up. Can't imagine how to ensure it was fair though.
Alternatively, might be an interesting math problem to calculate royalties for the training data used in each user request!
Such imperfect measures offer a compromise between "big tech can steal everything" and "LLMs trained on unpurchased books are illegal".
It's not just books but any tragedy-of-the-commons situation where a "feeder industry" for training can be fatally undermined by the very LLM that desires future training data from that industry.
Indeed the company should purchase the books. If they obtain copies in a process that violates copyright, then that's indeed a violation of copyright.
The current decision does not rule on the legality of obtaining the books without purchasing.