Google has as much ability to dictate their App marketplace as Apple. They've strayed away from such practices because they realize the dictation of applications begins to devolve into a very bigotry-riddled process which ultimately hinders development and innovation. Google could suffer greatly if one day Apple decides to ban or suppress their apps within the App Store due to competitive reasons.
Android devices don't even have to include the Google app marketplace. A manufacturer or network can put their own marketplace on the device instead, which makes it pretty much like every pre-iOS smartphone platform (at least in that regard).
I don't think that any single manufacturer has the leverage to tell the networks to stay out of the device OS and just be dumb bit transporters. I suppose Nokia could do it, but the idea's pretty unthinkable for those guys.
