It's most likely an email harvesting exercise to get people on their newsletter and details forwarded onto sister sites / affiliates, since you can't view the site without entering an email address and agreeing to spam.
It's a great idea - have "limited stock" to encourage the scarcity effect. I have no doubt they're losing money on a lot of those products; they must've done the math and figured out that it is a more efficient way for them to get subscriber info than any other method.
I would like them to have to announce how many items they have so to figure out what my chances are of getting something. e.g., I can't believe they'd have more than around 10-20 copies of Diablo 3 if they're selling it at $20 - I assume they're paying $40-50 for each copy.