As a former smoker, this doesn’t bother me that much. I don’t usually follow close enough to get hit by a butt that they flick out of the window (unless they flick it straight up). The litter bothers me a bit. It’s not like their car isn’t going to smell like smoke, so they should just use their ash tray. But when I see areas on the side of the road covered in litter, it’s not the cigarette butts that I notice, it’s the empty food containers, etc. As long as they’re not flicking them in areas where fires are likely to start or in pristine wilderness, it doesn’t bother me that much.
What bothers me, and isn’t really anyone’s fault, is having to breathe in the smoke that someone blows out of the window. When I’m following behind a smoker in relatively heavy traffic, I’ll usually do something to get in front of them, or I increase my following distance a lot and a few cars will usually get between me and the car in question. I don’t blame the smoker. When I was a smoker, I smoked in my car too. And it’s not like they should put their cigarette out when a motorcycle gets behind them. It’s just something that I try to avoid when possible.