Apple's recent restrictions on apps offering external subscriptions and content are starting to hit, particularly as these apps are starting to remove external links to purchase content that isn't offered through Apple's services. However, there's nothing that Apple can do about web apps - and that's what Amazon is looking to exploit to improve their Kindle service on the iPad, with the Cloud Reader.
The Kindle Cloud Reader, powered by HTML5, allows Kindle users to read their books either directly in Safari for iPad (it isn't available for iPhone and iPod touch yet), or by saving a bookmark to the home screen that will let it run as a full screen web app. From there, all the books that a user has purchased on Kindle will be readable, though some books are not yet optimized for the Cloud Reader. These books can either just be downloaded from the web, or can be downloaded to the device for offline reading, which can be done by long-pressing on any book in the library and selecting "Pin Book."
The most important part of the Cloud Reader besides being able to exist outside of the Apple approval process, is that it has built-in Kindle Store access. Users can browse and buy books from within the Cloud Reader, and start reading them right away. Compare this to the current state of the Kindle app, where the ability to buy new books, and any consideration of book buying is non-existent. It's like the presence of these books in the library is just a fortunate coincidence for the user. By pushing their Cloud Reader, Amazon can not only be platform-agnostic (though only Safari on iPad, Safari for Mac, and Google Chrome are officially supported, although it will be coming to other platforms in coming month), but can make it easier for them to sell Kindle books to users. Don't be surprised if Nook and other e-reader services start launching similar HTML5 options in the near future.