I think the puzzlement here over waving to cyclists vs. waving in cars or as pedestrians, is that pedestrians are so....well, pedestrian. Are cars are just all over the place. There is nothing "special" about them. But cyclists are a much smaller community. Hence, it's more akin to that small country road someone mentioned where folks wave. That is why many folks expect (hope?) that a wave will be responded to in kind.
Sure, many folks zone out on bikes. They're concentrating on their own thing, they're dodging obstacles, checking their speed, watching for traffic. A million things that take away their attention for another cyclist coming at them on the other side of the street. The brain may register them for a nanosecond and say "Not a car. Not a threat." and go on dodging glass. It's not a snub.
But when a cyclist who you have seen before, who does acknowledge you when you catch eye contact, then disses you when making similar eye contact because (perhaps) of the type of bike you have, that is a snub.
All things being equal on the rides, you can almost hear the snub. What is the line from that movie "A Knight's Tale"? - "You have been weighed, measured, and found lacking"?? Is that a fair or paranoid interpretation? Who knows? I don't want to crawl inside the minds of those guys.
Just a little respect, a little courtesy for all riders, is all that we can ask for. Because some day it will be you on the side of the road with a flat and no spare in your saddle bag, thankful for someone who stops and says "Got everything you need? You okay?"

