Comment by Glyptodon

Not to my understanding. To begin with, it's far from given that "rare" books are all covered by copyright. But if they are, it's at best murky: whether you destroy the original doesn't really have anything to do with what you're doing with scanned contents. The scanned contents themselves may be inherently a copyright issue, regardless of destroying the original. The actual trained model has separate arguments more in its favor, so if no scanned contents exist - IE the data is read once for training and not stored or saved, they have a better argument. But in that case the destruction is totally disconnected from copyright, as they'd be totally okay to rescan the material.