Cons of banning smoking in public places:
Demonization of smokers: prohibiting smoking cigarettes contributes to the stigmatization of smokers. Smokers are normal people, some of them smoke because they really enjoy the habit other because the fail to quit. Forcing them to smoke in private or only in smokers designated areas is a way to stigmatize them in society.
Comparative disadvantage: there are other annoying or potentially dangerous habits for which regulation is not so strict. For instance, in many countries it is allowed to drink alcohol in public places. Alcohol consumption is another great threat for public health. We often see people very drunk in public spaces. They may provoke fights and set a poor example for children. If we want to be fair, shouldn't we also ban drinking in public places? In some states carrying guns is allowed in public places. Aren't firearms more dangerous than cigarettes?
Taxes: tobacco is a great source of income for governments. Cigarettes are heavily taxed and the money collected from tobacco consumption can be used to fund research projects against the diseases caused by it or even education programs to raise awareness about the dangers of tobacco.
Pleasure: smoking has been considered a pleasure for centuries. Humans are often willing to undertake some risks for their health for the sake of enjoyment. It is accepted that people drink alcohol because the fun they may get from it, despite its obvious risks. Similarly, extreme sports are practiced by an increasing number of people regardless of the dangers they entail. If we want to ban tobacco because is bad for health, following the same logic, shouldn't we also ban fast food or sodas?
Freedom: public spaces belong to us all. Banning smoking in public areas means limiting individual liberties. Government regulation can become increasingly intrusive if we allow it. What is the boundary between the collective good and the individual freedom? There is a delicate trade-off and it is not clear where the red lines should be placed. What if after banning smoking in public places they decide to ban speaking loud, cracking jokes, or wearing a certain type of clothing? Can we objectively define what is necessary to ban or not?