I understand the point you are making. And everyone is already under suspicion, that's why there is a security check point to begin with. And a real terrorist won't be saying 'Haha, I have a bomb' to try and blend in. They will stay quiet. However it's possible a mentally deranged person could. The real problem is how our society reacts to all of it. If someone said it, and everyone laughed because they thought it was a joke, and then turned out not to be a joke, society would be calling for TSA agents' heads. We are fine with it until it bites us in the ass, and then suddenly we need to blame someone because OMG they didn't do their job and _who_ would be joking about something so serious.
It's just like getting pulled over for a traffic violation, you don't tell the officer 'Haha, I have a gun and shoot cops.. no wait I'm joking!'