I can't say that I agree with you that it's an idiom (well, because I've certainly never heard of it). It's a flat out socially weird thing to declare that you're not a terrorist. Social customs are hard to pin down, but if a passenger sitting next to me told the flight attendant that he wasn't a terrorist after the attendant told him to stop taking pictures (or to put away his cellphone), I would think of him as very odd.
I disagree - it's not weird at all. It's awkward; but the awkwardness comes from the fact that you're being confronted anti-terrorist rules. By their application they're basically saying: "I'm only doing thing because you might be a terrorist".
That's awkward, and it's entirely natural to want to deny the accusation. And sure that denial will also be awkward... but the problem is the assumption in the first place, not the denial. Everybody knows that the airport is a place of everlasting mistrust, no matter that it's not reasonable - and that you'll be treated unfairly because you're implicitly assumed to be a terrorist threat.
Everybodies thinking it (or trying to avoid thinking it) whenever the security is being particularly annoying, yet nobody is allowed to mention this thoughtcrime.
