Apple insisted that all apps remove the Store buttons from their apps - so it's not just Kindle that's affected. The easiest 'workaround' is to go to the Amazon site with Safari, find the Kindle Store then save that page as a desktop icon (use the curly arrow in the Safari menu and then 'Save to Home Screen'). Then, when you want to purchase a book, just touch that icon and you'll go straight to the Kindle store. Purchased books will still download into the Kindle app OK.
Apple made this change because they were hosting these free apps from Kindle etc and they made no revenue from any book sales. Apple wanted their usual 30% cut and the publishers baulked at this - so a 'compromise' was worked out.
Tim