They are destroying them because it’s easier to scan them if you slice the binding.

I didn't fully understand why but apparently there is also a legal reason to destroy the books, it makes it them less likely to be considered copyright infringement.

Not really. Selling the book onward does seem legally dubious but legally nothing (yet) prevents you from storing the book in a warehouse. obviously it’s cheaper to dispose of them.

And very hard to re-assemble after you have done that. They are not in the book binding business.