I wrote my master's thesis on a related topic.

Simplifying since I don't feel like looking things up right now:

Stresses reliably cause people to exhibit stress responses. That's it.

Nothing reliably causes any kind of deception specific leakage. To the extent that any do exist — which is doubtful at best — they exist in only small amounts, in laboratory conditions, with a very well established personalized baseline. (BDOs only even claim to do "environmental" baseline, which is completely useless.)

To put it another way, easy to understand:

Suppose I'm a detective. I get you into the police station, and like an idiot, you agree to talk to me. I sit you down and ask you: did you rape your sibling?

Now, suppose your sibling in fact was raped. And suppose you exhibit a wide variety of stress responses, desire to leave, avoidance of the question, etc.

Try to list how many reasons you can think of to explain any behavior you might exhibit, and how likely each motivating reason is.

Then consider what proportion of those reasons have anything to do with the chance that you in fact did rape your sibling, and the incidence of sibling-rape (which, I would add, is far more common than the incidence of terrorism).

That is what BDOs try to do, and your intuition on why it's wrong, in this particular respect, is correct.

(I like this example because so far I've not heard anyone be obviously wrong when they think it through.

Your intuition is totally wrong about a lot of other things though, like how good you are at detecting emotions. Answer: you probably suck, but much more importantly, your belief about how good you are has zero correlation with your actual skill.)

My reasoning was as follows:
-They wanted me to go through the body scanner since I was "randomly" selected;
-I can't go through the scanner for medical reasons;
-Therefore, I should be allowed to go through the metal detector, since people who cant go through the scanner should be allowed to go through the WTMD.

The TSA's answer is that if you can't go through the scanner for a medical reason and got denied the WTMD due to their random beep, then you get a pat down.

Which sort of makes sense; otherwise, any would-be terrorist selected would plead medical exemption.

Still annoying, though.