THE DISCOVERY of last week’s bomb plot originating in Yemen provided a bracing reminder that many packages that come to the United States aboard airplanes have not been scanned for explosives. Extremist groups still view civilian aircraft as a way to extend their reach into the United States, and the sophisticated US-bound package bombs that turned up in Britain and Dubai suggest a concerted effort to exploit gaps in the security system.The least the United States can do is demand complian...