>Amazon doesn't want their tablets to have the Play Store
Sure, but their customers do. It will be a lot less attractive option for some people if Google comes out with its own tablet, since it will have Google and Amazon's apps (setting aside the possibility of price difference)
I also partly agree with what habura said.
I think Amazon does care about the Google Play at bit because the developer fees on the apps go to Amazon rather than Google.
On the other hand it is also more about the services ecosystem that Amazon and Google are offering.
Take the following:
1. Cloud storage - Google Drive vs Amazon Drive
2. Digital content stores(books, movies, etc)
3. Music Subscription. (this came later, but still helps my point)
Amazon might've weighted the above and thought that if it has a monopoly of these services, people would go with Amazon rather than Google's services, since it will be available on more devices.
Note: This is all speculation, and even maybe as another user called it, a 'baseless conspiracy theory' so take it with a grain of salt. Longterm corporate decisions are hard to decode with confidence.