I think you might not understand the meaning of "unlocked". If it's unlocked, it's not tied to T-Mobile. Apple's website says, "The unlocked iPhone includes all the features of iPhone but without a wireless contract commitment. You can activate and use the unlocked iPhone on the supported GSM wireless network of your choice, such as AT&T or T-Mobile in the United States."
I see what your saying, but what is strange is that if you go to the unlocked section for the 5c and 4s, it's offers both a t-mobile unlocked and a sim-free unlocked. Why is there only the t-mobile unlocked option for the 5s. I use AT&T and want an unlocked 5s, but I'm wondering if the 5s still needs to be unlocked via t-mobile. Very confusing...nice job apple