they get them in stock way before, thats how we get prereleases normally. Someone nicks one from work and then copies it before they put them out on the shelves.
its all marketing. They can technically get in trouble for it though, but who cares... well apart from over zealous publishes (*cough* vivendi-universal).
I got a few games and dvds before their release dates, internet only companies don't have shelves so just send them out when they get them