Frightening stuff. What if someone is having a really bad day. Besides, since traveling these days is usually a nightmare to begin with, how could they even tell?
Learning to read people is a skill that security personnel ~should~ have. "Having a bad day" is easy enough to spot, especially in airports, and it's pretty normal. This is a technique that El Al has been using for many years, and they're pretty good at it. It's one of the best and most useful ways to "profile" people that have a different agenda than merely getting to a destination.
This is a good thing.
Who would be unhappy after waiting in line for two hours to get on a plane? Who would show fear as several Imams rant about the Great Satan? Who would be disgusted at an elderly woman in a wheelchair getting a body cavity search?
You mean like the cop who arrested a guy because he looked too buff?