The terrorists don't know whether the radiation detectors are effective or not, what fraction of the cargo is actually inspected, etc.
To keep them not knowing it is a deterrent.
To go and tell them, as this open session in the House did, and the NYT reported it, IS GIVING them concrete information they can use to avoid detection and plan a catastrophic attack upon us.
The only people who can't figure it out are the soccer moms on Wellbutrin. The only people with a motive to keep the population in the dark are the H0m3lAm3 "security" goons who want to keep their jobs despite being useless.