The legal idea behind it is that if you "copy" the book it's bad because now there are two copies and you "stole" from the author, but if you "move" the book to a digital form (and don't copy that outside of your organization), it's fine.

Given that they have to destroy them anyway, they're obviously also going to use the much cheaper destructive scanning (cutting off the spine and using a feed scanner rather than carefully turning page by page).