I think you're right, but the excuses probably vary by location. In my area (which has very low petty traffic law enforcement), people do argue that cops are all good, usually with appeals to "the rule of law". I often wonder if this is seen as identical to "the rule of law enforcement". Mentioning the compliance rate with the speed limit (or gun control, for that matter) just makes them angry and the discussion ends. Mentioning nonviolent crimes like cannabis possession is usually met with a claim that drugs require violence, followed by the same anger as they shut down. There's a lot of discordant opinions held about police, at least near me.
I suspect a lot of the counterproductive emotional reactions are a result of the fact that so much force is involved, but I'm not sure how to mitigate that.
I don't doubt your experiences at all, but my suspicion is that people are articulating their positions poorly, and their anger and defensiveness is explained by a frustration at their own inability to adequately articulate and defend their position.
Usually when I disarmingly unpack the "rule of law" issue with people, their argument is more akin to "if we crack down on police, then it will empower criminals" which is a different disagreement than whether all cops are good or all cops are bad. Their argument is perhaps more aptly described as "we have to choose between mostly good cops and mostly bad criminals and we'd rather take the former". They would rather police rough up a few protesters if it means dissuading rioters from burning the neighborhood down.
Of course, I firmly believe we can have our cake and eat it too--after all, better policing seems to exist in Europe, but it takes some faith and imagination, especially when I'm honest with myself about the ways in which we differ from Europe (wealth inequality, diversity, corruption, 2A rights, etc). I'm a pretty educated, articulate person who has thought a lot about these issues. I can certainly appreciate how the every-man would see things differently.
I think that means it's incumbent upon us to communicate a vision of things working properly that addresses our unique challenges, and I certainly don't blame people for failing to be persuaded by the riots, protests, and generally demeaning rhetoric.
I suspect a lot of the counterproductive emotional reactions are a result of the fact that so much force is involved, but I'm not sure how to mitigate that.