Back when the iPhone 4S came out, I purchased the AT&T version at full price at the Apple Store. The guys at the store told me that even though it was purchased at full price, that it was locked to AT&T.
When I got home, I removed AT&T's sim card and put in a Telcel (Mexico) sim card (while in the States) and connected to iTunes. I've used a T-Mobile, AT&T, Telcel, and Aio sim cards on it without any issues.
I got the "Your iPhone has been Unlocked" message.
This was purchased before Apple had the "unlocked" iPhones available. I used the following guide:
http://osxdaily.com/2011/10/25/buy-iphone-4s-without-contract-unlocked/
(Old, I know, but it may still work)
I'm assuming that the T-Mobile version of the 5s will work similarly, as well as the AT&T version (provided you purchase it at full price from Apple).
Apple must be selling the T-Mobile 5s as contract-free since it will come with the T-Mo SIM card installed, instead of having separate inventory for T-Mo versions with and without a SIM card. I think that they would prefer to not advertise it as unlocked to prevent resale of the device to other markets.