I've always thought that this is due to the fact that most people aren't capable of making the distinction between phone, providor and reseller, so most people who want a cell phone go to the provider directly, thinking of them as "the cell phone company".

The provider can then charge a premium to these customers, because even if paying extra, they'll still go directly to the provider. The thought of paying for a phone at one place and then paying for the service at another is too complicated. These are the same people who walk into a Cingular store and pay $40 for the name-brand Motorolla car charger, instead of walking next door to Radio Shack for the $20 knockoff -- or shopping around on the Web to find it for $10.

People look at products in terms of what they do. This carries over in how they buy things/services. They perceive the phone, service and even accessories as "the whole widget", and so when they go to buy it, they want to buy the whole widget from the widget people.

Anyone willing to do the extra legwork, even when it's just a couple of extra steps, will save a bunch.