Besides the possible copyright infringement problems mentioned elsewhere on this page, I occasionally write notes in my books. If I sent them an older book with a bunch of notes in it and got back a PDF that didn't have those notes, I'd be upset.

A potential compromise, if simply giving out the same PDF is legal, is them offering a big option saying "Our records indicate we have this book on file! Do you want us to immediately send you a PDF copy when we receive your book [potentially discounted pricing or even a percentage of resell profits], or would you like to wait for a scan?" So yeah, now they have to do something with the hard copies, either resell or donate.

Edit: No need to speculate, they claim to shred and recycle everything after processing.