I'm not sure that's right. Even if Kindle App has no purchasing option but accesses content bought on Amazon.com, it should still allow Apple's in-app purchase. Which essentially means, Kindle app is useless. See the excerpt below.

>> If an app lets users access content that they purchased via Amazon's website, for example, then that same app must also let users buy the same book via Apple's own in-app purchase system. If the app developer doesn't want to use Apple's in-app purchases to sell content, then the app can't access content purchased elsewhere either.