It is a bad idea as a matter of both law and policy to turn employers, either public or private, into arms of law enforcement agencies. If he can't, then he shouldn't be performing that day regardless of whether he's used drugs in his life. Maybe he was up all night or fighting the flu. What Mr. Spitzer is describing is just plain, good management. The score on a urine test is irrelevant.
Good management is practiced by supervisors in many organizations.
So, an employee who fails a drug test may not be impaired at all. Firing good, sober employees for something they might have done last Saturday does not increase safety. Drug testing may even decrease safety. Any experienced manager knows that a safe quality product and a safe work environment do not come from a demoralized, unhappy workforce. But this is exactly what drug testing produces.