This shouldn't be a surprise. The s7 is the same way. Also this is a choice from Samsung and not T-Mobile. All variants of this phone sold in the US are locked with no option to unlock. this has been covered in length on many tech and developer forums.
I would also note that the international variant may not have all the features onboard or in the firmware to take advantage of TMO's near Gigabit LTE which they are rolling out. They worked specifically with Samsung on it to ensure it worked for their network.