May be AT&T does not like having to fix all the customer complaints about all the hoop jumping their customers need to do to get those great deals.
Not to mention that a place like Amazon (or even Best Buy and Radio Shack) that sells AT&T phones can rely on many other sources of income. Sometimes companies will take a hit selling one product just because they know it builds customer loyalty over time and they can make up the loss with the vast array of other products and services they sell.