I'm a semi-occasional smoker, and I never litter the butts, there's usually a bin nearby or I keep it in my pocket and look for one ASAP because they stink awful. Additive-free tobacco smells a lot less though.

You can see other smokers doing it right when there's a bin nearby too, I've questioned them and never get an answer.

I think many people just live so thoughtlessly and carelessly, the rest of the city is dirty so why should they bother? It's really not much of an effort though.