As Pat Rogers always said, "Policing is regional".
It is only recently, through my participation in some of the good Internet forums populated by intellectuals such as you all, that I have come to realize the significant differences in law and procedure and custom from one state to another.
Around here (Wisconsin) unmarked cars have been common since before I started driving (which was 1975).
We get the occasional incident involving a police impersonator but I get the sense that such incidents are more frequent in other parts of the country