there’s a difference between the whole unhealthy food/fat thing and smoking. You do need to eat to live. There are people with good metabolisms that can eat stuff like that all the time and be no worse for wear, and most people can eat it once in awhile with no problem. Smoking is never positive to the health.
On the contrary, for people who think we should tax obese people, there are people who are fat for reasons other than laziness/gluttony. Genetics, metabolism, endocrine diseases, etc. There is no disease that makes you start smoking. Hence the difference, and reason why you can’t lump the 2 together.