[jballanc]

What you're referring to is a "soft target". You have to pay rather close attention, but terrorists have long recognized the opportunity presented by soft targets: malls, hotels, and yes, the TSA waiting line. Interestingly enough, it's the real security machinations of the US that have been preventing attacks of this form. Which raises the question further: if we already have real security, why do we even need security theater?

[DanBC]

In theory you need cheap security theatre to distract people from the very expensive, secretive, and sometimes unconstitutional (or nearly so) real security work.

In practice, security theatre turns out to be pretty expensive.