Paperback books that don’t sell are discarded so as not to compete with long tail purchases but hard cover books are expensive enough in terms of production and distribution that the end up getting discounted instead.
When I lived in the UK, Fopp was a great source for them (along with discount films and music.)
If you're a bookstore and a hardcover doesn't sell, you have to return the whole book to the publisher. If a paperback doesn't sell, you just have to rip off the front cover and return that to the publisher.