It's not clear what this means:
a) Amazon has to provide a way of purchasing content via the apple store (and presumably can do so at 30% markup). Meanwhile, books that you purchased to read on your computer, kindle, android phone, etc. is still available to you on your iDevice at no additional charge.
b) Amazon is not allowed to transfer content that the user has already purchased. Users have to repurchase all content through the apple store and the only advantage of the kindle app is if you like the reading experience and syncing with your existing devices.<p>If it's (a), then ok. I wouldn't purchase through them, but some people might do so for the convenience (instead of starting safari and purchasing it that way).
If it is (b), then as soon as there is a credible android tablet, then it's likely to be goodbye to the iPad for a number of book users. The advantage of Amazon is that while the content has DRMd for the end user, they can read it on a variety of devices. Also, the selection is excellent (not perfect, but for light reading, pretty good). I don't see Apple winning the content war on this one. I suppose Amazon could make a web-based kindle service (again, DRMd) that would could be logged in through one's amazon account and used through Safari. The experience wouldn't be as good, but it would still let people read content on an iDevice and screw Apple.