I always thought this was strange too...

An interesting take on "the publishing industry needs to die" that lists the industry's many sins, including this, which always struck me as odd:

...as a matter of practice [they] tear the covers off of books, ship them back to the publishers and THROW AWAY the books, because actually shipping them costs too much. Yet, these same stores will not stock even one print on demand book (the wave of the future), because they are not "returnable" -- even when they rarely return a book to the publisher in the first place.