It doesn't matter why these people carry guns; that is their business. What matters is that they are trained to carry a weapon and are an asset to the flight crew if they are on board.
I noticed the usual mention of how guns are a problem in the US compared with the UK, where guns are banned. Yet the usual blindness is showed toward the rapidly rising and out-of-control crime rate in the UK and Aus, where the government successfully removed all legal guns, leaving only the criminals to be armed. Aus and the UK now have the industrial world's highest crime rate as a result. One crime that is particularly heinous is Home Invasion robberies, often involving physical harm and death. The rate of this type of crime is way higher than it is in the US, where the crim knows he may be facing an armed householder.
What type of crime is similar to home invasion? Why airplane hijacking of course! Methinks that the authorities are setting us up!