And then, those movies companies simply resell those same movies when some occasional re-orders come in, or via their own DVD music clubs or whatnot. Great secondary markets for movies to be sold in. If only videogames had a secondary market or some way to leverage old stock like that.
Or they come back a few months later in our $5 dump bin...
Anywhoo, with paperback books, most retailers tear the cover off, send it back for credit and destroy the rest of the book (recycle!). There's no real secondary market for paperback books - although some paperback books *are* the secondary market.
Same with movies. Most "new release" DVDs are the secondary market (theatrical release being the primary).
Ideally, if you know your game is good and it's going to sell (or it should sell), there's no reason not to offer to buy back unsold copies (Zack and Wiki would have been a good game to do this with... Giant half-pallets displaying the game at Wal*Mart would have sold more copies. How about Boom Blocks?). If you know your game is trash (Dogz, etc...) then yeah, it makes sense not to offer to buy back unsold copies.
The idea of "we want you (retailers) to buy our games - no matter how crappy we make them - and you get to shoulder all the risks" is half the reason the distribution of games is so screwed up right now.