Exactly! These devices are worthless, especially if the person is amoral or is a "professional liar." Given the OP's target audience, it would be terribly ineffective. The real value to these tests is that people will often voluntarily spill their guts because they are intimidated by the technology. Since this spilling of guts was not obtained during the actual polygraph exam itself, it's all admissible evidence.

Of course, you could make it go off, like a wand deliberately run too close to shoe eyelets, and see what reaction you get.