they don't exactly have to articulate 'why' so much as they do have survive the attempt - if any - to prove gross negligence. Police are protected by sovereign immunity, so if they kill an innocent, or half a dozen innocents, as long as it cannot be proven in court that the individual officer's actions were grossly negligent, they get off scot free. Not careless, not accidental, not stupid, not belligerent, and not negligent - grossly negligent. Shot somebody by mistake or on purpose... shady circumstances - not gross negligence. Chasing down a guy for speeding 8 mph over the speed limit, exceeding the posted limit by 60 mph, blow an intersection, kill a family of 5 - nope, not gross negligence (Fairfax county, VA).
There's a "Monkey see, monkey do" phenomenon. Most of the time you see a cop it's on the highway. Chances are, they're speeding too. In fact, the only time I see them going the limit is when they are specifically looking to poach a speeder by pacing. When people see that kind of behavior, as well as hitting the lights so they don't have to wait at an intersection like everyone else, it breeds contempt. I think there's a similar "too quick to go to the gun" mentality among law enforcement these days - in general - I've not personally known an officer to behave that unprofessionally. Truth be told, my personal experience in VA lends me to believe they make great efforts to not behave that way. Still - Monkey see... monkey do. Some of the news reports, videos... Then there's the taser. Nope, now they can't shoot them until they've tried to taser them first. Yup. Let's taser folks for a traffic violation. I have a better idea - firing squad for cops who think tasering for a traffic violation is ok.
Of course, the other side of the coin is that while the individual, heck maybe even his whole department may be the pinnacle of professionalism, the neighboring departments may not be, and may behave irresponsibly in public. Monkey see, Monkey do.
As far as the IACP report and the head shots... Let's face it, that's the reality of the world we live in today. These martyr bombers are inhuman, and I'd much rather see them endure scores of years of agonizing torture as opposed to their instant deaths, but the former is not practical, the latter is.