How tho?
Ever sunday at the flea market, I see thousands of books that are rotting, hoping for someone to buy them or at least take them home, so the seller doesn't have to pack them for the trip back. Just the other day, there was a whole bin of books in front of a shop, offering them for 50 cents a piece. They will be destroyed anyways.
Unless they are buying and destroying really old, rare books or important small-print books, it is not much damage. It is not like they will buy "all copies of all of the books", just one. And its just that the data in physical print most likely hasn't been used for training, so this can help you find more unmined quality data. Nobody is stealing your books, preventing you from buying more or destroying all copies of a single book.
And some of these books would rot out of circulation or be destroyed anyways. Some people throw away 80-100 year old books on the regular, as they might just be unimportant to them or the world in general. And once the last copy is thrown or rots, that book will die forever. This way, it will live forever instead, scanned and trained on, conjoined with the rest of our knowledge in a magic machine.