Maybe the revenue here is small for them, but the possibilities and relationships it opens to them with app developers are good, and could help their app store indirectly.

For example , they are currently working on paying developers through in-game ads.Another source of payment could be t-shirt discounts, or t-shirt marketing services.

EDIT:and let's not forget - if t-shirt game marketing becomes popular , and most such t-shirts would be for games in amazon's appstore , that would create a powerful marketing tool for amazon's appstore.