Pulp fact, even. I happened to be in a bookshop the other day with a senior editor from one of the big publishers. I was looking at one of the books on the shelves, which hadn't just got a few handling marks - it was seriously browser-battered. Knowing that bookshops can send books back and get their money refunded, I asked if the publisher even paid them for books that so obviously couldn't be reused. The response was a bit of a surprise to me.
In principle, I was told, the publisher could refuse to refund a book that was in a really bad condition. But in the end, the chances are they would all be pulped, so it didn't matter. Returns weren't usually sent back out.
I suppose I imagined craftspersons in the warehouse, carefully restoring returned books to the piles waiting to go out. Giving them a quick polish with a chamois leather before they lovingly restored them to stock. But, no, it's heave-ho into the mashing machine.
Perhaps it's just me, but this seems an awful waste.