Which is a sensible clause for Apple, so that Amazon can't go and say "all books bought on the Kindle device are 50% off what you pay when you buy the book on the Kindle for iPad app" to drive device sales.
You have it a bit backwards, the kindle is there to drive ebook sales, not the reverse. Apple only added the clause when they added the mandatory in app sales with the 30% tax - if amazon had any intention of doing something like that you'd presumably already have seen that behavior on the iphone or pc or osx or android.
