This actually came up on a local facebook group I am on - a motorcycle rider was complaining that the harley riders never wave to him.
Somewhere in the thread someone said basically they will wave to anyone "except scoots, because they aren't bikes." Someone else countered with "I ride a Harley, and wave at everyone, even scoots, because two wheels is two wheels. I don't tell them what is good for them to ride."
A third person chimed in with basically, if he had a scoot, he would ride the h8ll out of it.
By the end of the thread, three themes emerged:
1. won't wave at any scoot (minority)
2. wave at anyone on two wheels
3. wave at anyone on two wheels if they are female. otherwise only wave if waved to
4. will only wave at scoot drivers if they wave at first because holy crap they think we all crazy for driving around on wheels that small and if we want to risk trying to drive on these pothole crazy roads and wave, we can take that risk ourselves!!! (This was actually shared by many and sort of amused me given the things these people cop to on the group)