The vendor issue aside, this sort of random response is exactly the right way to do it. Screening is all about raising the risk for someone who actually wants to do harm, which minimizing the negative impact on the vast vast vast majority of people who don't.

So, if we go back to 9/10/01 screening (metal detector, shoes on, as many liquids as you want, etc.), but randomly assign people to an extra layer (NoS or patdown) of screening, it raises the risk profile to someone who wants to smuggle something through (since they won't know for sure if they have to prepare for WTMD or NoS) while reducing the overall burden dramatically.