We trust them with the plane don't we? Why not a gun.
I agree with your logic Outerlimit. I wish it were as easy as that, but it isn't.
While I fervently believe in everyone's 2A rights to carry whatever and wherever they choose, I don't disagree with the additional training required to carry in the capacity of an airline pilot. An airplane flight deck is a highly structured environment in which safety is paramount. Changes to that structure have to be carefully developed, refined, and examined to avoid disastrous consequences. Carrying is an additional responsibility with procedures that have to fit into that environment. If you carry in your daily life (and I hope you do) you may have had the benefit of formal training. But even if you didn't, you probably already have procedures and training that you developed on your own, though you might not think about them in that way. Think of the four rules, for instance, and how you apply them to what you do. It probably works just fine for you. However, formal training to carry in a highly structured airline crew environment is a must.
Certainly the requirements for formal training limits participation. A lot of pilots have already had their spare time and spare income slashed to zero and zero respectively, and this program demands both. Many feel they already put in enough effort to be given command of a $35M airplane with a couple hundred people on it, and being involuntarily granted additional responsibility for no additional compensation already happens with sufficient regularity in the airline business. I imagine that's why the TSA believes participation will only reach the 16.5% they project.
Bear in mind it's a federal program. Any mishap is going to result in the kind of attention no Fed program administrator wants. So there are a lot of hoops to jump through, to make sure the .gov covers its rear in the event the inevitable happens like it did with US Airways 1536.
A friend of mine, former LA city cop, told me they had a number of police cars with bullet holes through the driver's seat, about where the seat belt attaches, i.e. of NDs went on there. Not much of a problem when all that gets hurt is a cheap vinyl seat and some dirt beneath it. People are skittish enough about all that aluminum flying over their heads every day though, let alone all those federally sanctioned lead launchers... hence the hoops ya gotta jump through...