Judging by Lawdog's writing and others, cops get into all sorts of weird and bad situations in their careers. Allowing them to sue sets up a very bad precedent. Also, cops generally are provided protection by their city and their union and have a general presumption of respect by the public. What is left of that if they start suing?
What are the cop's actual damages anyway? His health care is covered. He can't say his job is peaceful with no chance of violence.