We're not wasting money trying to prevent something that never happens; we're basically paying career insurance for the powers that be (Tm) in case someone actually /did/ blow up a plane.

They'd look awfully bad if they hadn't done anything to counter the threat, no matter how marginal (Oh, and in fairness - there /are/ people who want to blow up planes, though as a first approximation one could do worse than estimating their number at zero...)