We've touched on this subject before, but it looks as though Apple and its lawyers have been busy behind the scenes with that in-app subscription policy. The news today is that not only Amazon, but also Kobo and Barnes & Noble have finally blinked. All three have removed the contentious button within their e-book reading app that linked buyers directly to their respective e-book stores. That leaves Apple's iBooks as the only iOS e-book reading app with single button access to an online e-book store.
If you like your Kindle e-books on Apple's hardware, there's no need to worry, however. All of this brinksmanship was over fact that Apple demands a cut of content sales when purchased directly through an app running on an iOS device, such as an iPad or an iPhone. Removing the button from the app doesn't mean Kindle, Kobo or NOOK app users are prevented from buying e-books, it just means that they have to manually navigate to the respective e-bookstore in Safari and buy the title that way. Essentially, it's two clicks (assuming you have the website bookmarked) instead of two, and then you have to flip back to the e-book reader app from Safari. Once purchased, the e-book will then auto-magically become available in the app, through the user's account.
It's a minor inconvenience, true, but if you feel that put out by the extra click and are determined to have that single button access, you can always turn to Apple's own native e-book app, iBooks.