I think you're confusing "we don't sell it" with "it can't be used". With AT&T and TMobile, you can bring any unlocked GSM phone and, as long as the frequencies are compatible, it will work on their service.
The MotoG supports both TMobile and AT&T's frequencies. But you're going to have to do this backwards from how you're thinking it should work. You're thinking "I'll choose a carrir and then to to the store and buy the phone from them." The way it actually works is, "I'll order the phone from Motorola online and then I'll order a SIM card from my carrier that I can stick in to it."
The MotoG supports both TMobile and AT&T's frequencies. But you're going to have to do this backwards from how you're thinking it should work. You're thinking "I'll choose a carrir and then to to the store and buy the phone from them." The way it actually works is, "I'll order the phone from Motorola online and then I'll order a SIM card from my carrier that I can stick in to it."