A kook suspected of threatening the President is being charged in part with illegally possessing body armor. So basically, the police are allowed to protect themselves from homicidal miscreants but we as citizens are not allowed to protect ourselves from homicidal miscreants which easily include errant police. Since no one is harmed if an individual owns a bullet proof vest since the vest alone cannot be used as an implement of violence, a free citizen would be allowed to own a bullet proof vest.