But seriously how did taking a photo get connected to crime?
Watch any crime show and the killer usually has a stockpile of photo's on a blackboard complete with a diagram of how to do the kidnapping/murder/whatever. It makes it visually easy to show the viewer what the main antagonist was up to, so it now gets associated with being up to no good.
Having said that, I once had my car break down on a busy freeway and since I had to wait for the repair service to come pick it up, I found a safe spot between the two directions of freeways which was not accessible to pedestrians and sat down there with a book to read to pass time.It Didn't take long for the police to show up to question me what I was doing there either, so I guess the police just looks for unusual behaviour.
Taking pictures of schools, armed transports, military installations or kids does seem to fall under those categories. On the other hand, those stereoptypes can also be used to your advantage, try pinning on a badge (like those from the Umbrella Corporation for example) and carrying a clipboard and pen as if you're marking something and people will really try to pretend not to notice you in order not to get asked any questions. The one downside is that if you do it in stores, you get people asking you where they can find a specific product : )