An employer should be able to fire someone for what ever decisions they make.
This includes their decision to smoke, dye their hair, drag race or be a homosexual.
Each are choices.
Exactly. What most people making the slippery slope argument here fail to realize is that each additional condition an employer demands reduces the available pool of workers a little more, forcing them to pay more or compromise on other factors. Some companies, like Scotts, may decide not to hire smokers, while others will likely take advantage of the availability of this group of workers and hire them. In a free market, there's no way smoking, or any other minor factor, will prevent someone from finding a job if he has the ability to be productive.
If said description is made after that time, de facto as it were, I'd run the employer through every court in the nation, and cheerfully (and win, too).
Only the Regress gets to change the rules in the middle of the game, because Mr. Madison didn't think to put in a clause in the Constitution to stop them from doing so.
The rest of us live by any reasonable interpretation of law, and for better or worse, under what might be described as status quo ad hoc at the time of events.