People who lose or are unable to gain jobs because of drug testing are a bigger drain on the economy than lower productivity from drug use.

The War on Drugs has not been shown to reduce drug use. Hence, drug legalization would have no effect on this.

If a worker does shoddy work, they can be fired.

The hundreds of thousands of people currently serving time in prisons are missing a lot more work then any conceivable losses due to excessive drug use.

Correlation does not prove causality. Those likely to do drugs on the job are just as likely to be the type of people who would miss work and perform poorly whether they were on drugs or not. Furthermore, drug use can actually help job performance by correcting things like anxiety and chronic fatigue where the War on Drugs has made obtaining adequate treatment through legitimate medical channels impossible due to the unwillingness of a lot of doctors to prescribe controlled substances, even where the need is real.

unproductive employees, drug users or not, often experience difficulty holding jobs; society has a perfectly fuctioning mechanism to deal with burnouts. However, most drug users are responsible in their use and are of no negative consequence to society.