The "unpredictable security" mantra is now a catch-all when you challenge the TSA.

In the thread on shoes in or out of the bins, I asked why there wasn't a directive sent to each airport stating the shoes are either all on the belts or in the bins and then everyone would be on the same page. A retort from a TSO said that it was designed that way for the unpredictability factor.

Right.

Because Al Qaida operatives are sitting somewhere thinking, "If only our shoes had been IN the bins, we would have succeeded in blowing up the plane."