I don't think it's as clear cut as you make it. Like I said, I have no legal background, but I do read the news
What about the teachers who have been
fired for having making legal adult films years before they were teachers? Or that woman who was fired for innocent pictures of her
drinking alcohol on vacation? And I think some Utah college basketball player (working for his full-ride scholarship) was punished for having premarital sex recently.
And the woman who stated abortornot.com, a website to vote on
her decision for her own reproductive choice, was fired for being a "grave threat to the reputation of the company" despite the fact that all her activity took place after work. They also forbid all of their current employees from being friends with her on social networking sites.
This all seems unfair, but it does not seem unprecedented to restrict employee behavior outside of work, at least it seems to be norm right now.
A spokeswoman for the U.S. Equal Employment Opportunity Commission said they saw "no problem" with not hiring smokers from where the federal government is concerned. Now, of course the federal government can be wrong, but the fact that they won't challenge it is an indicator of the current interpretation of the 4th amendment.
Many states have made legislation to specifically protect smokers. Only a handful go so far as to have law that protect people who "engage in lawful activity or use legal products outside the workplace".
The EEOC spokesperson did specify that the testing could only take place after the job offer was made.
http://www.ama-assn.org/amednews/2011/04/04/bisd0405.htm