Point of order: You can actually install Amazon App store apps through the Amazon (Store) app that IS available via Google Play. And yes I know that is a horrible sentence to parse. I think you do still have to have "third party sources" turned on for your Android device, but the Amazon App store is not 100% required.
Apple doesn't offer access to its content for non-Apple set top boxes and it doesn't offer even an option of non-Apple hardware for much of anything, but people still eat Apple's shit with a smile on their face.
I own most of the major STB devices aside from Apple's, but in my opinion the larger, not-Stick FireTV is the best of breed option. It works with services I'd actually want to use. It's responsive in its UI and can be used for light gaming. It's open enough to support non-Amazon apps if you're willing to sideload them. If Amazon wants to push its position in that market,I think I'm OK with that.
If Google really wants to push the issue, killing Amazon's API key for Youtube access on FireTV devices would probably be a strong bargaining chip, but having used both Chromecast and Nexus Player as well as Google TV hardware, I think STB devices are still an afterthought and besides, there are still plenty of places to buy them.