This isn't totally new - it's been done in Michigan, as well. Basically, they're dealing with nicotine like what it is - a drug. And like other drugs, they can ban them. Just like if you discovered an employee was an alcoholic. The issue becomes the health care costs associated with it that the companies have to bear. And some jobs have policed weight for a long time - airlines, fire departments. Police departments need to - I've seen some seriously obese cops in this city. What the heck are they going to do to protect me - clearly they can't run. They are not "fit" to perform the tasks of the job, and that should be regulated.
As an aside, being gay is not high-risk. Engaging in certain activities, whether straight or gay, may be high-risk, but orientation is not. I think outright bans on smoking in the workplace are wise, from both health and safety perspectives - and I'd bet the families of the 9 firefighters recently killed because of careless smoking at work would agree!