Wow, I'm new here and what a great thread to start off on.
I spent over 5 years in law enforcement and it was that career that really broadend my love of photography. I took courses in Crime Scene Photography, digital imaging, etc.
When I worked the streets, if someone came up and asked to take my picture, or pictures of my car, I let them. I would inquire as to what kind of camera they had, etc. As long as it didn't hinder me from my job, I found it to be another way to interact with my community.
The problem is, there ARE people out there who want to take pictures of cops for nefarious reasons. It only takes one of them to keep a cop on his toes. If I'm dealing with a suspect of any type, I'm not really in the "smile please" mindset.
Respect the crime scene, respect the cop and his boundaries and you're more than likely going to walk away with all of your stuff and sleep in your bed that evening.
Sure, there are jerk cops, but after reading this thread, there are jerk photographers too....