I imagine if Amazon had to reimburse developers for any non-zero amount this would be bad for business. It sounds like they're in this "free app a day" thing for the long-haul.

It also seems to me that what the developer is getting for this is a free day of advertising. So if someone tries your app who normally wouldn't have because it's prominently displayed on the top of the screen and they like it they might buy the next version or other apps from you.