You're thinking about things from the TSA's perspective, think about it from the traveller's perspective: they are waiting in line, knowing full well that all of this shoe taking off and public cavity searching is unnecessary, and now they have the added knowledge that if they complain there is a man standing nearby with a gun.
I'm thinking about it from the perspective of a TSA agent as a human being who may want to be able to defend themselves against attack like anyone else.
Why should travelers not be armed then? I walk into an airport with no weapons at all, no ability to defend myself from an attack. In fact the purpose of the checkpoints, in theory, is to ensure that travelers are unarmed.
One practical reason is that no one else can parse the motives of someone walking armed into an international airport until they start shooting or don't. At least cops and (maybe, maybe not) the TSA agents are supposed to have them, and be trained to use them, and trustworthy not to start murdering people.
So.. either everyone can carry everything they want anywhere they like and we all just hope for the best or else an attempt is made to restrict weapons only to those known (theoretically) to be lawful actors.
