The City is full of tourists and locals taking pictures of everything all the time. I never even got a funny look shooting candids with an slr, people are used to it.
I can't imagine that many people will bother nor have the time to report people taking photographs.
The funny thing is that the imagined terrorist who needs to take pictures would do so with a point & shoot or with a portable phone, and with a couple of friends. He will do what everybody else is doing. Nobody will rush to him. Nobody will report his behaviour.
For 99% of people (especially including policemen) "suspicious" behaviour means "somehow different". If you do something different than you are a suspect.
If you take the same picture of the airport with a tripod, you are a suspect.
If you take out and use your spot lightmeter you are a suspect.
If you study your composition more than 2 seconds, you are a suspect.
If you carry a bag or some suspiciously full (of lenses) pockets on your vest, you are a suspect.
You are a suspect for no other reason that you do something "different", regardless of how your behaviour might be connected to any crime.
Just to give you an example, I have been considered a suspect for looking at night constellations (you have to reach some place without streetlamp and cars headlamp, suspicious behaviour) and for looking for/at foxes near where I live (you have to drive your car very slowly at 2 or 3 o' clock at night, what could be more suspicious?).
The ordinary policemen is not trained to anything else than "different behaviour" = "suspicious behaviour".
That's easy because "different" is understandable by any dog whereas "suspicious" requires turning the brain on. Besides, policemen like simple rules (however senseless) because they feel their bottom is better covered.
The result is that we are LESS secure with this kind of thinking and this kind of behaviour.
To give you another example, I was once in front of the synagogue with a tripod. A motorist stopped for a second and really stared at me with hate (I could not believe eyes could tell "hate" so well). A policemen came immediately after and prevented me from taking pictures. During this, everybody around was taking pictures with their point & shoot.
I was there again a few days ago, and I took four nice pictures. Free hand.
I could go there twenty days in a row and take twenty series of pictures without the least problem. I am sure if I go there with a tripod I am immediately questioned.
The average terrorist does not need to be a genius to overcome the zeal of the average policeman for sure.