We have to draw the line somewhere; if not here, they'll push the line further, and further. The TSA has no motive to do a cost-benefit analysis: the force pushing them forward is risk, no matter how small, but the only force pushing back against them is how much budget they can get out of the American taxpayer. The saliency of terrorism fears overpowers analytic reason in too many people.

A quick search indicates there are over 87,000 flights daily in the US alone; that puts the number at over a quarter billion flights without a successful terrorist incident in the absence of these scanners.