I guarantee you your phone did NOT come rooted from Motorola. Root access is something you need to hack the android OS for. Not even Google's Nexus devices come prerooted...that would be dangerous as malware could easily infect such a phone.
Your device did likely come unlocked however, which means that you can bring it from carrier to carrier without issue. Is your device not working? Did you root it yourself? If you didn't root it, and it's not rooted, and it's not working on your device, then that would be the first I've heard of it not working on a non rooted device. What model is your phone? If it's an older model, it might be that your phone does not have the technical specifications required for a smooth experience.