There are legal obstacles most air carrier managements will shy away from: Additional company time for recurrent weapons training and law enforcement qualification; Passenger liability issues if an innocent passenger is shot; Airframe insurance issue if a bullet damages expensive on board equipment or flight controls/hydraulics; Concealed weapons permits in foreign countries; Air carrier liability for death or injury of a person in a foreign country by one of its armed pilots; Multiple complex bilateral agreements vis-a-vis firearms protocols and regulations. Additionally, unless the firearms remain in the cockpit, as United's "stun guns" will, pistol toting pilots in foreign countries will be individually responsible for weapon security whether at hotel, at sightseeing or at restaurants.
Overall, professional law enforcement and the carrying of a loaded weapon in public across state lines and across international borders is in itself a full time job. And the professional pilot who wants to take on those additional duties will neither be a professional cop nor a professional pilot.
Besides, given the events of Sept 11th, there is a new mind-set; It's about crew and passenger survival instinct! And given the new inflight security training, reinforced cockpit doors and bulkheads, the probability of another hijack in the USA is practically zero. To be sure, passengers fearing for their own survival will no longer sit idle during any air rage incident, nor will passengers allow any terrorist or other mentally challenged person to attempt to storm into the cockpit. The curtains in the aisles will remain open.
