I use a single bud, right side only, open-air sideways earbud from Sony (cut off the left side). I listen to books on tape and NPR. I can still easily hear cars coming 1/2 mile away, just as far as without it.
I think listening to music isn't too smart, most people want to turn it up loud enough to be an enveloping experience. I listen to my audio just enough that I can hear the words. It doesn't hinder my ability to hear cars at all (I've tested it, I can hear them exactly as far back, or at least, as close as I can tell, with or without).
Don't assume anyone with an earbud is being an idiot, though probably some of them are (I've ridden by joggers where **I** could hear their music.
Also, as has been said, some cars are pretty quiet. I used to unintentionally scare joggers in my old Ford Probe. Even in a quiet subdivision, if I was going < 10 MPH they couldn't hear me unless I drove over some gravel or something.