1. Microsoft's not likely to let Amazon put Amazon put Windows 8 on the next Fire for free.
2. Microsoft will be running the app store for Windows 8 tablets, not Amazon (BTW, do we even know what the in-app purchase rules will be for Windows 8? If they're at all Apple-like, that's a dealbreaker by itself).
3. Windows 8 is likely to have more demanding/expensive hardware requirements than Android
Amazon totally forked Android and ripped out all of the google bits, all of the default UI and made it into their own OS.
It's Android in that it will run Android apps but that's it.
In all other ways, Amazon's Android is competing with Google's Android, which is one of the reasons Amazon don't describe it as an Android tablet unless they are talking to developers.
It's actually a really interesting strategy, although from an Open Source perspective it's always a shame when companies fork successful projects for their own goals.