Don't be naive. Do you actually think AT&T is driving a money losing promotion for a phone system that has just 1.5% of the smartphone market share? Yes it's the hardware mfgs that are selling the handsets to the carriers but they're not driving this promo. The 800lb gorilla in the room is Microsoft and they're putting heavy money into increasing their market share. Whether it's subsidizing it through the carriers or the hardware mfgs, there's no doubt MS is the driving force here.
MS has big plans for their ecosystem; look at Windows 8, Windows Phone, Microsoft Stores. Everything points to them building the ecosystem for the long term. To do that you sometimes have to lose some upfront money to win long term. They want to increase their market share!