T-Mobile and AT&T have drastically different pricing structures, so comparing them is a bit difficult. T-Mobile only charges per line, with a discrete service plan for each line, and device costs stand entirely apart from that. AT&T charges data as a separate service from the per-line cost, and since they still offer contract plans, the contact subsidy premium is expressed in the line cost. Thus, when you bring your own device (or buy one at full price or on an EIP), you receive a line discount.
T-Mobile's "discount" is invisible because it doesn't offer service on contract, so there's no plan without a discount.