In fact, the T-Mobile version of iPhone 5s is unlocked. Look at the explanation below it, this sentence below states it clearly that it's unlocked:
"When you travel internationally, you can use a nano-SIM card for iPhone 5s from a local GSM carrier."
It is NOT SIM-unlocked for domestic use. There is UNLOCKED - totally SIM unlocked, and international-SIM unlocked - works ONLY outside the USA.
Check the same section of text under the "UNLOCKED and CONTRACT-FREE" iPhone 5C.