Presumably because there is no punitive tax on food, flip_flop.
Smokers may or may not be net contributors to the NHS. I believe that is arguable but is anyway beside the point.
It is the effects that smokers have upon non-smokers and in particular (as far as this question goes) upon employees of pub companies who are exposed to their fumes that were of concern when the legislation was framed.
Your citing of the �volenti non fit injuria� defence (that is, that the employee willingly undertook the risks presented to him by his employment) would not hold water. It has long been a legal assumption that whilst employees may understand the risks involved in their jobs, of necessity they usually have to work and therefore do not assume these risks voluntarily. In short, employers cannot avoid their responsibilities by suggesting that the employee was willing to undertake hazardous tasks, the risks of which could be lessened.
The examples you quote of risks involved in other activities are not appropriate. To take the van driver, it is not possible for the employer to reduce the risks posed by other road users with whom his driver may collide. All he can do is ensure his own driver is trained to as high a standard as possible. Similarly the police and army personnel may fall foul of third parties who may cause them injury. These risks can be reduced by suitable training, but cannot be eliminated.
Smoking in enclosed places is different. The risks to staff can be eliminated, and indeed should have been by employers without the need for legislation. They were not, so hence the need for the ban.