Though Sasquatch beat me to it (as usual), I'll go ahead and respond again...

Pronography and being black aren't illegal. Those ARE legally nobody's business. It's not because of the drug specifically that employers don't like this; it's because it's against the damned law. If you commit mass murder in your off hours, would you expect your boss to high five you when you walk in the next day? No! You're a damned criminal, nobody who needs to maintain any kind of public image would want anything to do with you. If weed was legal, I doubt they'd test for it, because then, as long as you weren't showing up to work blazed, it wouldn't have any impact on your work, in which case it would be invasion of privacy if they drug tested you. Seeing whether or not they're employing criminals is not. And at this point in time, weed is illegal, so if you test positive, you are in fact a criminal, criminals recieve no special priveledges, they're punished. Don't like punishment, don't break the law. Simple solution.