Because (I suspect and as I mentioned previously) it reduces the insurance liability when one of the staff / cleaners tries to sue because they have contracted a smoking related disease. [1]
I agree there could still be 'special' places which were obviously only of interest to smokers (like 'Smoking Clubs') where members of the general non smoking population would not need to be in contact but I still suggest the above para is to do with it.
I'm not sure you could get a member of staff to waive their rights either? "I hereby sign that if I become ill or die from a smoking related disease from working here I (or my family) won't sue the company" ?
All the best
T i m
[1] This is probably folk law (or will be discredited as such) but I heard there was an instance of a smoker who contracted lung cancer who then tried to sue her employer for 'encouraging her to smoke' (because they had proved a 'smoking room' on the premises). This could be a reason why many companies *don't* provide such facilities and further don't allow smoking on their entire site.