I interned at a daily paper back in college and we had a print hanging in the darkroom (yes it was the film days) of an enraged cop staring into the camera, pointing and yelling. The chief staff photographer thought it was pretty funny - had apparently been too close to the action to suit the cop, and the two had become rivals at accidents and crime scenes.
My take is that cops (representatives and enforcers of the inflexible, often inadequate rule of law) have to contend with us photographers - inquisitive, open-minded, boundary-pushing types. We are two inherently opposed groups. Course that's painting with a broad brush, but I figure there will always be tension between the two groups.
This is especially so because cops, given the very hard job that they do and the high stress level they live in every day are generally going to have a pretty short fuse. As we have seen from the recent U.S. atrocity in Afghanistan, if you subject human beings to years and years of extreme violence, stress, and fear for their lives, some of them are going to crack to some degree or another. I think it's important to remember that cops are people first and foremost, and the majority are doing the best they can to do things that most of us would fear to do. That said, I also feel it is a journalist and artist's calling to continue to ask questions, cross lines, and go the places the sworn to duty aren't allowed to go.
