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?