> Amazon's devices don't support the Google Play store...

FireOS devices aren't certified by Google and so, they do not come with "Gapps" (including the Play Store) preinstalled. Even if they were, Amazon might have some reservations about preinstalling Gapps (which run with higher than normal privileges), effectively letting Google get hands on its user's purchasing habits.

All that to say, this is a business limitation not a technical one.

See also: Google's iron grip on Android: Controlling open source by any means necessary, https://news.ycombinator.com/item?id=6582494 (2014).