Personally I think crApple keeps a tight rein on ATT in order to keep the competition light on their "exclusive" network. Obviously this is in the process of changing as we get closer to the release date for the iphone to possibly go elsewhere imo.
The Android Market started as a competitive edge to equal the iphone. Motorola probably has some kind of deal with VZW right now. Plus, wouldn't you make higher end products for a company that doesn't sell what you're competing against?