back to the "car analogy": it's the responsibility of the owner to comunicate to the DMV any "non-street-legal" mods she does in her car, and it's the DMV/PD responsibility to ensure that "non-street-legal" cars do not drive in the streets.
IOW: the FCC (in the US) could go after people who modify their phones so they are not anymore in accordance with its regulations, and operate them in the network. Others should be AOK.