Drug testing for job applicants is simply a money saving policy. If there is a great number of applicants for a job, screening out the drug users BEFORE skill testing and interviews saves resources. Also, it's based on the fact that a druggy PROBABLY can't do the job as well as the non-druggy AND, perhaps even moreso, based on the fact that a druggy MAY cost the employer grief in various ways in the future.

It is simply good business sense.

I think that applying the, "If me, then why not them?" arguement is flawed in this instance.