Re: From outside the US
>U.S. law allows employers to insist that you be drug-free
Its a bit more subtle that this. If a corporation gets federal funding in any way -- in other words, does business with the Federal government -- then it has to have a drug-free workplace policy. This includes drug screening of new hires. Needless to say this has spun off a whole new industry.
Its a Regan era "War on Drugs" thing. I personally only know one person who's been caught by it -- a colleague was laid off and then re-hired as a contractor from Intel. He failed the drug screen -- marijuana, of course -- and despite the best efforts of the managers involved had to be terminated.