I can see how a group ride could see it like this but single Harley riders don't greet you back either (unless I'm on a Harley or something they in passing might think is a Harley).

I'm guessing you are from the US, which I'm not, but I have never seen this in other bikers no matter the brand. Sure Ducati guys often hang out with eachother at a meet, as do other groups, but it's not like they don't greet other riders.

Heck, I even wave (end get waved at) when I drive my Alfa Romeo by both other Alfa drivers but also people in classic cars (and a few Ducati riders even). Totally different cultures than Harley riders, which I think is sad.