I expect Amazon to sell more Kindle Fire devices than any other manufacturer will sell Android tablets. From that perspective, yes, it does make sense to support it and get your app into the Amazon store.

I read someone comment that side-loading software would be allowed, but I haven't seen any official word. I would be surprised if it was, and I was fully expecting Amazon to make the devices compatible only with their own store. It doesn't make as much sense to allow side-loading since this is first and foremost a content delivery platform for Amazon. The content is what Amazon has over other random device makers.

Imagine Amazon coming out with a number of other devices also built on this OS. And all locked to Amazon's own app store and all with a completely Amazon-based UI, devoid of all native Google apps. Something closer to Apple and iOS. I can see such a platform doing well. Well enough to do more business than the other Android device makers. Enough business that Google might kick themselves in the ass for all the lost revenue.