Dave,
Don't think of it as a "CS" thing since as some have said it's simply the overall BMW ownership today. Even those who have purchased classic BMWs often are immune to enthusiastic greetings/encounters. My association with BMW started in 1972, and back then essentially everyone was an enthusiast of one sort or another, and if not they usually got sucked into the fold and convinced to join the CCA. It was simply not possible to meet another BMW on the road and not exchange headlight flashes. The majority of the cars were of course 2002s with an occasional 1600 with E3 sightings very rare (2500, 2800, Bavaria, 3.0S) and E9 (3.0 CS) sightings extremely rare.
A few years ago I came across a Malaga 2002 coming at me, and I flashed my headlights in appreciation only to have the owner look at me like I was an idiot as we slowly passed each other. I was driving my E39 M5 in shining LeMans blue, but I suppose the 02 dude figured I was up to something.
A few days later I saw the car again, from behind, and SOB it was a 2002tii even. Over the following years when I would see the car again, I always flashed at it...never to be acknowledged.
Regards,
Chuck