Just to play devil's advocate: Apple pays for the infrastructure to host and distribute the Kindle app, which then proceeds to circumvent the payment system by offering the app for free and charging for the content from outside the Apple store. While this set up is clever and 30% is a bit of a margin on in-app purchases, it's not as if Amazon has the moral high ground here. It's providing a paid service through Apple…
We could always look at it from Amazons POV. They are providing the content that makes iOS devices more desirable for consumers. Why isn't Apple giving them a cut of each device sold?