Yes, this is a result of copyright laws. The other commenters are wrong/uninformed.
If it was up to the companies training LLMs, they wouldn't destroy the books: It's a waste of company resources, it's needlessly destructive/evil, it generates bad PR, etc etc. There are essentially zero advantages, other than it is what is required under US copyright law (or at least, it is what their highly paid lawyers believe is required under US copyright law).