Unfortunately, Google's contracts with OEMs are secret, and the newest ones the public has seen are five years old. But Google has previously used terms that outright barred a manufacturer from having any Android devices without Google Play, if they dare partner with another company for even one model. And in order to get Google Play, the terms they must agree to also require preinstalling a large list of apps, and setting them as default.
I suggest you look up the Android MADA agreement (the ones on the Internet are from 2011), and then consider that by all rumor, they've gotten worse since then.
Maybe the biggest problem is the fact that these agreements are secret, and not subject to public review. I doubt Android's support would be so widespread if the public knew about these contracts from the start.