Honest question: Why?

Isn't that up to those who hold the stigma?

I'd have a stigma against dog walkers who left their dog crap at the park, even if I found it was most dog walkers. (I don't think it is)

It’s because the stigma is based on the “binge drinker” label and its implication of irresponsible levels of drinking, while the official definition of the label itself specifies a level of drinking that many would otherwise not deem an issue. eg. three restaurant-pour glasses of wine during a three course meal.

I think the difference there is that even if most people left dog crap we might still have a consensus that that was a bad thing and that people shouldn’t do it.

Stigma doesn’t exist simply based on how many people do something.

Would you say the same about cigarettes? Or about drunk driving?