You are correct - but I also vividly remember a message playing as I went through security in a US airport a couple of years ago saying that things like inappropriate jokes etc. could result in arrest.
Of course, it's not like a psychopath could lie convincingly so that he could proceed with his evil plans, so I guess they can't actually let anyone in, ever.
Sure, sometimes someone is suspicious because he's actually up to no good. But that doesn't mean detaining people on various subjective suspicions is a good idea.