Only tangentially related, but I am continually astonished by the absence of any sort of social norms surrounding cigarette butt disposal. I have (otherwise environmentally conscious!) friends who smoke, and I've personally witnessed them flick their cigarettes into gutters and on streets. As a serious question, without judgment: are there any smokers on HN who can explain the phenomenon?
Since the rest of it (paper, tobacco, and ashes) is easily degradable, I think the natural assumption is the butt is degradable as well, and the notion that every other smoker seems to think so isn't helping.
So to answer the question: I suspect it's simply a lack of information. As a data point I'll add that I stopped deposing them right away a long time ago, because I suspected they might not be easily degradable, but this is actually the first time I've read this is the case.
If I'm correct I see no harm in correcting friends who smoke in their behaviour. They are simply unaware.