I found myself waiting for a red light behind another vehicle this week. That vehicle's driver had his window down, his elbow on the sill and was holding what was left of his cigarette between left thumb and forefinger. I knew exactly what was going to happen: one last drag on the butt and flick, away it went into the ditch.
What's more, if I looked to my left I would almost think that I was stopped next to an ashtray. The ground was covered with discarded butts left to smoulder into oblivion by people who probably didn't stop to think of the potential for destruction that their action represented.
I agree, between October and April of each year there might not be a lot of risk, but once the habit is formed it will continue through May to September without a second thought.