The issue here is not just that Amazon might want its own app store, a reasonable desire. The issue is that the current Android market really sucks. Google does not have good expertise in the curation methods that an appstore needs; right now, you have two options browsing the appstore: you can look at top, all-time sales. Games that have been out for two years top these charts, not surprisingly.
Or, you can look at the raw feed of 'newest'. In games, that would be 64 underwear puzzle games, three things in Japanese, and a tech demo of rotating lines, controlled by some sensor or other.
Google's traditional approach to this sort of problem is search, but search does not work well here, and there's significant market opportunity. Hence, Amazon.