I can see both sides of the picture. From what I heard, it costs more money than it saves, but I think it is more about principle.
I had to take a drug test before becoming employed, so it seems fare. But then again, that is 2 private parties entering into an agreement. I don't know the legality however with regards to government required drug tests.
I would also support it more with the stipulation that when someone tests positive, they are given the option of treatment instead of just being turned away.