Every carrier makes negotiates a deal with Samsung. No matter if it's ATT, Sprint, T-Mobile, or Verizon. Depending on the deal they get, since ATT and Verizon are the Nations largest carrier, they have the ability to purchase more units in bulk therefore gets their prices slashed, giving their customers a nice little price break. T-Mobile and Sprint are not in the same position as ATT or Verizon, therefore their purchase power is significantly less than them. Therefore, their prices are higher, and would need to charge their customers more for the phone without cutting their profits.

This case is similar to how Apple's iPhone was only exclusively sold at ATT stores, until the recent couple of generations when it was released for Verizon.