I usually wave or say "hello" when I pass a cyclist or pedestrian.
If coming from the rear, I consider a greeting a way to help them become aware of my presence.
I have had a couple of people in cars yell a greeting out of the window, then speed off, and I've wondered if perhaps they were pedestrians that I passed at 20 MPH with a greeting without allowing time for them to respond. Is that equally rude?
I don't worry when people don't respond. There are much more important things in the world to worry about.