Why wonder? The answer is abundantly clear if you follow the news. If a book is copyrighted under U.S. law, scanning and destroying counts as a format conversion which qualifies it as fair use, so there is no need to negotiate with copyright holders. See Judge William Alsup’s decision. If Anthropic did not destroy the books after scanning it would have not won the lawsuit, and scanning would be illegal. If a book is already out of copyright then of course they do not have to destroy it afterwards.