What case do they really have here? SIM Unlocking is already been deemed LEGAL in the USA, whether the company selling the device gives permission or not. If you're buying the phone, it is yours to do what you wish. The company sells devices at a lower price (still more than they purchase them from the OEM), in exchange locking the SIM to only use one provider, in hopes that you will spend more with that company who then will recoup the full retail value (an artificially inflated price) of the device over a period of time. But you're not obligated to do that. You can top up and pay into the prepaid plan and ask for an unlock code if you chose, or you can take that device and use other methods to unlock it.
If AT&T doesn't want these devi...