Simcox was prosecuted for possessing a firearm while on lands of the Organ Pipe National Monument. National monuments have more restrictive regulations and state laws concerning firearms do not apply as they do on national forest lands.
It was Coronado NM, not Organ Pipe. He was only a few feet inside the national park, but a rangerette was after him and busted him. That area, like most of the other fed lands around here, are closed to law-abiding citizens, either proverbially or literally (e.g. Buenos Aires NWR). The illegals have controlled those areas for at least 15 years.