My explanation will not be popular here on HN, but I'm never one to shy away. Especially when asked directly.
Buying a book, buying an audio CD, or buying a DVD/Blu-ray is granting the holder permission to read,listen,view that product as a single instance. You can lend them out, but that's all you're really allowed to do with them. The text,audio/video is not owned by you to do with as you please. People obviously do not like that, and argue making copies/backups is their right. Maybe that's acceptable, but we can agree posting them on torrents and sharing in any other manner from a copy made from the thing you have is not.
Saying that, training a model on someone's copyrighted text is not part of the agreement of the usage of said text whether it's a copyrighted magazine, newspaper, or book. If the people doing the training reach out to the copyright holders and get specific permission to use their copyrighted material in such a manner, then go ahead. The fact that people feel like they can do anything without the common courtesy of asking for permission is troubling to me that we've lost something as a society. There's no acknowledgment that someone has created something by their own work so that the creator can do with it as they please. A large portion of people believe that because it was created they deserve/should be able to/etc do what ever they want with someone else's creation. Including getting paid for derivitave works from the original creation.