Quite a few years ago, there was a chain email going around that warned drivers NOT to flash their lights at other vehicles because gang members would flash their lights and if you returned the flash, they would shoot you. While this was complete hogwash, there are still a LOT of people who believe it to be true.
I have noticed that the practice of flashing your lights at other vehicles has diminished greatly since that chain email started circulating.
When things like this bother me, I try to remember my golden rule: I do nice things for others because it makes me feel good; not because I need recognition/acknowledgement that I've done a good deed.
If flashing your lights at others makes you feel good, then do it for the feeling - not for the accolades.