It comes down to these values. I wrote this paper because I value the right to privacy more. Employers, like anyone, have been effected by the Reafer Madness Movement. The government pushed massive amounts of misinformation throughout communities and schools, and I don't believe that employers are well informed enough yet to dictate what drugs will harm the workplace. The only effective way to select workers is to evaluate their performance on the job.
Drugs can actually improve performance.
Aspirin relieves pain, allowing a worker to continue. Marijuana will actually make some people more alert. After intensive testing, someone I know can solve the Rubix Cube 20 seconds faster when stoned. Stimulants will keep workers productive at the end of long work days. If the negative effects of drug use begin to show in the worker's performance, their employer has a number of options for dealing with it.