If they can't tell, then what are they checking for?
Opponents of tyrrany.
This is about compliance, not security.
The same thing customs agents are looking for: lack of eye contact, nervousness, something out of the ordinary, something that may set one person apart from the crowd. Plus presence equals deterrence.
Many of the same people who bitch and moan about these inconveniences are the same ones who will scream "Why didn't Bush do more" if a bomb goes off.