[quote name='imacjenn']Why can't they just lower release day prices to $40 already? I'm really tired of these credits and immediate price drops.[/QUOTE]
The credit guarantees them future business from you, and most people have a hard time sticking to a strict budget ($20 in this case) for an order. Instead, they'll buy one or a few things that total substantially more than $20, and Amazon makes some profit on that add'l money.
It also involves the time value of money. AMZ gets your $60 now, and you may not use the credit for months. In fact, many of these credit deals have expiration dates that I would hazard to guess that 25% of people who get these credits never use them.
Bottom line is that AMZ wouldn't offer these deals if they didn't make money overall. If you enjoy the low cost of AMZ and are disciplined w/ your future order, you can benefit from being in the same pool w/ people who don't ever use the credit--they're the reason the credits are out there.