Terms of Service – Compliance
See the difference between the two screenshots? The one on the left has the “Shop In Kindle Store” button that takes you to the Kindle store in Amazon.com to buy books. The link was removed to comply with the terms Apple has set forth. I’ve never used that link to buy books. Of the sixty or so, free and paid, books I’ve bought for Kindle, all have been done via the Amazon.com store on a computer.