I have to say that I agree with the rule. Think of it from the other (nefarious) side. Yes, you can find drawings on the internet, but if one were planning something, well, there's nothing like firsthand intelligence. And no, that shouldn't stop anyone from taking what are obviously family photos, but documenting precise layouts and protocols does raise potential risks. How do you know for sure that 9/11 was "an event unrelated to picture taking over 6 years ago"? The
9/11 Commission report states that the terrorists took a number of surveillance flights in preparation for their attacks. Do you know for a fact that none of them had cameras? And yes, maybe they could have gotten the same info with careful sketches, but a camera would be much easier, wouldn't it?
We have the same rule on the NYC subway. Of course, no one cares if some tourist takes a photo of their family on the subway, but I certainly sit up and take notice if someone starts taking multiple pix of the car/doors/emergency equipment itself with no friends around. Actually, this happened about a year ago - a closer look revealed that the photographer was wearing a conspicuous press pass around his neck, probably because of this very issue. I won't speak for Londoners and Madrid-ers, but I would guess they'd do the same.
As for the OP, the FA definitely should have taken a more professional approach and simply informed him of the rule. Yes, maybe our civil liberties are being trampled and common sense is not being used enough. However, I respectfully submit that some of the comments others have made herein are one-sided oversimplifications of the current situation.