>>It has nothing to do with the law. It has to do with an "at will" employment contract.
Given the "at-will" provides for discharge "for any reason – or no reason at all", it appears the guy should have quit smoking; however, in the business climates I'm familiar with, the action wouldn't stand. The "at-will" appears to be a creature of the 19th Century. I came across this page which speaks to At-Will Employment in some detail:
http://www.rbs2.com/atwill.htm (target=>'_blank')
Right to work and "at will" employment are characteristics of freedom. Forced union membership and "just cause" are the signature of socialists using the iron hand of government to interfere in the affairs of private business. Communism and socialism are creatures of the 19th century too. Collectivism and government enforced altruism are putrid doctrines that should be avoided. They fail every time they are tried.