Just 3 days ago, we were having the exact same debate with my rider mates. Some of them GS owners, haha.
We couldn't precisely name the source of this behaviour, this lack of etiquette, but it remains a fact: If you blow the horn-nod you head - wave your hand, chances are you will be ignored, lets say, 75% of the time, if the rider is on a BMW. From BMW to BMW, it is a different story, my friends say.
If the guy in question is a jerk, that doesn't mean I have to be one as well, so I wave every time I can, even when I know I'm most likely gonna be ignored.