I can answer that. Because it's become so offensive to even "smell" smoke, that most smokers drive with their windows down to cut down on the "smell". They don't want to offend anyone. As for the butts out the window.... the most annoying thing to me is the fast food bags and drinks tossed out at 70mph. I never see any "butts" along the road... yet I sure see the cleanup crews going after the McDonald's, Wendy's and Jack in the Box bags/containers. I guess it's all in what you focus on, eh? :)
I understand about the smell, but I wouldn't think that smokers would all, as a class (and nearly all of them do this, it seems) vent the smoke out like that. I would think it would be less offensive to confine the smoke to your own car, rather than let it escape for others to smell. I guess smokers don't really like the smell that much either. I remember dating a girl whose room mate smoked. She didn't seem to like the smoke from her own cigarette, as she held her hand far away and waved any smoke away from her own face (often into the faces of others she seemed to ignore).
I do see other litter, and that bothers me too. I can't believe you don't see cigarette butts on the road. Next time you are at a stop light, especially on a road with a median, take a look. They are everywhere, it seems (of course, it could be because I live in the Winston-Salem, NC area, where smoking was, for many requires, nearly a requirement of living here.).