I think a lot of the issue stems from the extremely aggressive tone cops use. Dogs react poorly to people acting aggressively towards their owners. When I was a kid, the PSE&G guy came to read our meter. Only my sisters and I (my oldest sister was maybe 14) we're home, along with our dog. The dog was barking at the meter man, but keeping her distance. He then yelled at my sister to shut the dog up, or control it, or whatever. Bam! In an instant, the dog ran over and bit him.
Look and listen to how cops talk to people (being as they are, increasingly militant and aggressive), and it is no surprise that they frequently feel "threatened" by the dogs. It is actually the cops who are typically threatening.