Scott,
I'm a Cingular dealer so I'll give you the run down.
The phone cost let's say $300. When you activate there is a commission given for let's say $200 for a 1 year and an extra $50 for a 2 year deal. So doing the math a $300 phone will cost the company with a 2 year deal $50. So they need to sell it for more than $50 to make some money.
These numbers are kind pulled out of the air but it all kind works that way. You service no matter who you buy it from is still Cingular.
Hoep this helps