It really can't. But lets not get into protected classes (gays, women, minorities etc.). Lets say that I own a company, and decide that all employees will get on a scale, and be weighed. In the ineterest in having more good looking people, I will fire all employees who are more than 10 lbs over their optimum weight/height requirements. You and a bunch of others refuse to get on the scale. You're all fired. Ya think I may be sued? Damn right I will and I should.
Now, smokers will never be deemed a protected class and I'm certainly not an employment lawyer, but these people may very well have a cause of action for at the very least, emotional distress.