I wave at people when I see them. Especially kids.
I have probably been guilty of being rude but it's not my intention. Usually, if I am driving, I am going somewhere and thinking of the call or listening to the car radio/computer. If not, I am looking for traffic violations (tags, etc...) If I am stopped, sometimes I am watching something, writing or on the computer clearing my call. If I am doing a traffic stop, all my attention is on my radio and the car, period. If they look at you and don't return the wave, well, maybe they are just plain unfriendly.
In the rare instances I am just kicking back, I usually catch people trying to get my attention and don't mind chit-chatting or saying hi.
Sorry you feel slighted, if you are ever in my town...I will buy ya a donut!
NorCal