I think it's a combination of not wanting to deal with approval and Apple (their Kindle app theoretically could be dropped for not using Apple's in-app-purchase

I think it's a combination of not wanting to deal with approval and Apple (their Kindle app theoretically could be dropped for not using Apple's in-app-purchase system [which is pretty much impossible for an app with 5000+ IAPs!]), the rumored color Kindle running Android (which would likely use the Amazon Android Marketplace), and their various efforts in the general Android ecosystem including the Marketplace.