The frequency does not have to be outside of the FCC limts for this to happen. It's simply poor Radio Frequency (RF) shielding in either the cables or device itself. I'm a ham radio operator and we deal with RF interference all the time using totally legal frequencies. In fact RF interference is why many well designed devices will have a cylindrical object in the connecting and / or power cords. That device is a ferrite choke and it's purpose is to minimize the induced current that results from nearby RF fields from affecting the device.