I think Amazon allows Hulu and Netflix because they're huge Amazon customers. Well, I'm not sure about Hulu. I do know that Netflix uses something like 1/3 of internet bandwidth, or internet usage, or whatever the measure is, and they use Amazon Web Services. Amazon sure wants to encourage them. Also I think they were using AWS long before Amazon started offering a competing product.
All this is off the top of my head. I'm not sure of any of it except that Netflix and most other large providers use AWS.
I'm sure part of the reason Amazon doesn't provide Google Play is that they like keeping their devices simple. I think it's true that their appstore is more difficult to use than Google Play but having both on a tablet causes a lot of confusion with respect to which source to use for updates. I had a couple of Android tablets with Amazon's app store as well as Google Play and for that reason I removed Amazon. It just caused a lot of problems.
I do have Google Play on my Fire 7" and I don't recall many problems there but the only Google apps I use are ones that Amazon doesn't provide and Amazon updates it's own apps blindly on the Fire. It doesn't do that on Android tablets. If I downloaded a Google app that I could also get from Android that could be a problem.
Amazon could very easily have prevented our installing Google Play easily but they didn't so it's hard to think they really care much.
By the way, I also think that Amazon totally fails at keeping the Fire simple. But I think it's what they want to do and they try to do. They just don't quite know how. The term that comes to mind when describing the Fire's interface is "user sadistic".
Barry