Probably a combination of that and three other important things: a) compromise on things like this in order to gain bigger freedoms, namely, the App Store - ca

Probably a combination of that and three other important things: a) compromise on things like this in order to gain bigger freedoms, namely, the App Store - carriers are usually getting a cut, but not in this case b) poor consumer protection in the US - carrier unlocking is, at least in some countries, required by law c) (related to above) young, immature mobile market in the US