Uh what do you mean you can't easily find it anymore? Search engines still exist (perhaps avoid Google unless you have to use it) and neither bookstores, nor the library, are out of books.

The reason they can do this with books is that there are so, so many books that nobody else wants to buy. Like so many that you can just buy books by the foot as a decoration. What books will you get? Nobody knows because nobody gave enough of a fuck to write it down. They're only valuable for LLM training because they're new data and they're only "rare" (as said in some articles) because there wasn't enough demand to print more.

And destroying the books means there's no extra copy of them, so for the purpose of copyright law, all is good unless an LLM straight up recites a passage from a book.

Honestly that whole thing might end up being net neutral or even positive, as the paper can be recycled, instead of sitting around as books nobody will read.