It's a summation. Go pick up litter for an hour or 2 in your neighborhood and categorize the rubbish you collect. When I do this (large sample) the results are: ~80% tobacco, 14% fast food waste, 5% alcohol waste, 1% other.
Point being, many smokers litter. My thesis after a lot of public service is they do this because: they don't care about themselves, so why would they care about anything else?
Oh, I agree with you in this regard, but your take was... overtly simplistic in a crucial, key detail.
Concerning litter presence in general - as much as the poor manners the distinct absence of the bins strongly amplifies the problem. At least in my country (pretty large western country) - most of the public spaces (streets, etc) lack ANY sort of the bin, and while it's easy to tuck the plastic wrapper from the food and take it home, I'd say people are much less inclined to carry a stinky cigarette but or leaking can for a couple of miles.
Addictions exists. To stop smoking is HARD. Nicotine addition us on par with benzos, prescription opiates or amphetamines.