TSA's crazy screener-testing: giving "bombs" to regular passengers to sneak onboard?!?

Bruce Schneier notes that the TSA is apparently using "plainclothes bomb-testers" who approach regular fliers and hand them fake bombs and ask them to sneak them through security to test the screening process. This is a bad idea:

Someone please tell me this doesn't actually happen. "Hi Mr. Passenger. I'm a TSA manager. You know I'm not lying to you because of this official-looking laminated badge I have. We need you to help us test airport security. Here's a 'fake' bomb that we'd like you to carry through security in your luggage. Another TSA manager will, um, meet you at your destination. Give the fake bomb to him when you land. And, by the way, what's your mother's maiden name?"

How in the world is this a good idea? And how hard is it to dress real TSA managers up like vacationers?