Smoking is a personal choice, refusing to hire a smoker is also a personal choice.
So is deviant/gay sex, but for some reason you can't discriminate against potential employees who engage in high risk behavior, even though they could easily milk your health plan with the cost of thier behavior. Personnally, my observation is smokers are more focused on the job. The perceived cost to the employee is minimal when compared to aids. One other note, they aren't the ones who normally catch and spread the colds/flu that visits an office every year. I would assume that an employer who mandates no-smoking in hiring is performing a urine or blood test to know. If he knows you smoke, he also knows many other things about you.