the kindle app store doesn't have all the apps that the google store has.
i guess it's for the kindle models with lcd screens that are just regualar tablets with amazons own android version.
samsung also has an alternative store, don't recall its name. also on the nokia x platform there is a store with android apps (but not all of them)
as kindle is a growing platform it may be of interest for some users
It all depends on the company, its development team and its relationship with the corporations that own the platform I'd say. Some like Sirius XM refuse to support Windows Phone no matter how emails and tweets they get. Some like FitBit have come onboard and really display their support for WP on their website.