Are you sure thats accurate? T&C from TMO's site for the 6s promo states:
"Limited time offer; subject to change. For well-qualified buyers. Eligible paid-off smartphone trade-in, lease, $15 SIM starter kit, & qual'g service req'd. $20+taxes/mo ($24+taxes/mo for 6s Plus) for 18-mo lease after monthly bill credits. Must remain on qual'g service in good standing for duration of Lease term. If you cancel wireless service, remaining payments (of up to $$27/$31/mo) become due. Must return device in good condition or exercise purchase option at lease end. You are responsible for insurance/repairs; insurance recommended (required for CT customers). Not avail. with Contract Freedom (device pay-off) or other trade-in offers. No device security deposit req'd. Late/non-payment fees may apply. For upgrade, you must trade-in eligible device in good condition at participating T-Mobile store & upgrade to eligible device on lease; up to 3 times per 12 mos. Participating stores only."