If the FBI investigates 500,000 people and prevents a single instance similar to the photo below, is it worth it?
http://11even.net/wp-content/uploads/World-Trade-Center-2002...
There have been plenty of attacks stopped http://en.wikipedia.org/wiki/Terrorism_in_the_United_States
There is always the possibility of something even worse happening. http://en.wikipedia.org/wiki/Nuclear_terrorism
A notable incident from 2013: http://abcnews.go.com/Politics/fbi-foils-plot-build-strange-...
I'm sorry if this sounds like fear-mongering, these things do happen though.
What I'm talking about is an 80 year old guy who is well documented for professionally photographing public art.
When they saw someone taking pictures of that piece there could be a couple reasons: A) he's scoping it out to come back later and lob an RPG at it, B) he's working on the second edition of his book you can buy on Amazon.
They don't need to talk to the guy to find out which one it is. That's what I'm saying.