While many people are criticizing the action, I can see the merit in it. The Amazon Appstore has a little over twice as many Apps as the Windows store, and cross platform compatibility is actually what saved Apples desktop, and may have helped save Blackberry.
We all know apple is doing pretty well now, and that Windows compatibility did not make their hardware irrelevant in the eyes of consumers, rather the opposite, it gave them the option of buying a Mac, at a time when the market was Windows dominated (As it still is).
Microsoft is also reported to be announcing a universal app platform – something that will likely also benefit the likes of Blackberry, and make the job for programmers easier when it comes to covering Blackberry 10, Android, iOS and Windows, in one hit. And because the Google Play, Apple Store, Blackberry World and Windows Store all only run on their native devices, the cross compatibility of android apps from the Amazon Appstore, does not really dimish the power of the idea, especially when Windows 10 devices are the one set of devices that will be able to also run full Desktop Applications, and do so in a seemless switch from touch to desktop mode.
It also seems probable this is a temporary situation, with Google likely to pull the plug on the open source aspect of its Apps, given that both blackberry and Windows will be able to run them soon. Between that and the closed down apple system, Microsoft universal Apps system, might be a safer bet for developers.
I can see this adding to the value of using Windows phone or Windows tablet in the short term, and the universal App system in the longer term, but of course we will have to see tomorrows expected announcement, and the years to come, to see how this really plays out.