Why are Smokers Exempt?

I'm no "liter policeman" (like Steve Coleman), but I'm curious to know why smokers are exempt from the traditional public courtesy of giving a hoot -- and not pollute-ing.

Now, I'm not an over-arching "anti-smoker" kind of guy. Do I think it's a gross habit, that is clearly life-limiting, and makes you smell? -- Yes. But as long as you are a respectful public smoker, than I'm not going to judge you. I could elaborate on what this entails, but I think I'll save this for another post. Generically, though, it's just about being conscious on the "second hand" effect that smoking has on people other than you...

So, getting back to the subject, why is it socially acceptable to flick your cigarette on the street or outside you car window, but not acceptable to just drop your empty Starbucks cup on the ground in mid-step? Did I miss the "cigarettes are bio-degradable" lesson in 2nd grade Science class?
Furthermore, if the cigerette is still lit when you flick it -- that lovely smell is going to linger for the people behind you to smell. I mean, what if people had this mentality for public toilets? (cuz we all know how good public toilets smell)

Anyway - I don't really have the balls to call someone out on it on the street, so thought I'd vent this way. If you're a smoker, please think twice before flicking your cigerette butt on the street. Put it out with your foot and treat it like a used tissue. Thank you.