Because it's possibly to smoke without being a regular smoker. I smoke like 3-4 cigarettes a month which is negligent in terms of health compared to a lot of things everyone do on a daily basis. It's not moronic as the effects are inconsequential and it's not affecting others if you're mindful about where you smoke, and it has nothing to do with self esteem.
Not smoking/smoking being bad is not their opinion. The fact that anyone who ever smokes lacks self esteem, is not considerate of others and are fucking morons, those are their opinions in this case.