Actually, this is exactly incorrect. The reason Amazon couldn't include any Google Play Store compatibility in Kindle or Fire products, for example, is because in order to do so, Google illegally requires OEMs to also bundle other apps... like Google Play Books, a direct competitor to Amazon's Kindle. This is a practice called "tying" in antitrust law.
The fact that Google doesn't allow OEMs to pick and choose what apps are included is the subject of an EU antitrust investigation, and Google has already been prohibited from doing so in Russia. It's also illegal under US law, but Google has an incredibly significant amount of money invested in the right US politicians.