I can't really see how that is true since t mobile and sprint both have and embrace the nexus program. In fact the t mobile moto x is the XT1053... Same exact phone as the gsm Dev edition save for the words on the back. Hell, if woven white was available through moto maker, I could go build a woven white back and black front phone and have the words "Developer Edition" put on the back and it would simply be the exact same.
I don't really understand the warranty thing with t mobile since a 32gb moto maker x is the exact same price and exact same model with the same process for unlocking. For Verizon I get it because they already have a locked phone policy, but t mobile doesn't. In fact, T-Mobile says they carry the phone but it isn't a T-Mobile branded phone (which is why they don't have Wi-Fi calling on it).
As well, functionality wise, if you want a nice looking unlocked moto x for at&t, you moto make a t mobile version and unlock it.
At least Motorola has a no questions asked return policy. I unlocked mine and was able to return it. Of course I flashed back to stock (huge pain compared to normal fastboot) and relocked it prior to returning. One thing they don't mention is they pay for return shipping, contrary to their website.
Sent from my Nexus 5 using XDA Premium 4 mobile app