This doesn't make sense. This isn't like Facebook or iOS where you are a sharecropper working their platform and they can pull the rug from under you at anytime. The platform here is the web, Amazon doesn't own the web, any company you build on the web could attract competition from anybody else. Building your business on top of Amazon's top-notch infrastructure doesn't give Amazon any advantage if they decide to compete against you. And nothing would stop you from moving off of their stuff if it did. I don't know why anyone would have the "understanding" that Amazon wasn't going to compete against them. Amazon's cloud actually evens the playing field, there's nothing stopping you from building a bookstore and competing against Amazon using their own servers against them.

In short, your complaint doesn't make sense.