Does anyone approve of drug testing on the job? [View All]

Recent governmnet furthering of drug testing as well as my own upcoming preemployment drug testing reminds me of this.
What does drug testing have to do with anything? For preemployment testing, if a potential employee is interviewed extensively and has good references, couldn't an employer be reasonably certain that the employee does not have a work affecting drug problem. Since the applicant may have plenty of time to prepare, won't most applicants prepare.
What is the purpose of random drug testing? What business is it of the employer if the employee uses drugs off of company time? As far as reports about drug testing reducing problems at work, couldn't an employer just fire problem employees? What good does it do to test and fire good non problem employees?
Doesn't making drug users unemployable make them more likely to turn to other crime? Doesn't stable employment minimize the impact of mild drug use for most Americans?
Even if drugs are really bad and negatively impact employment, why are marijuana users, who are often impacted the least negatively, targeted by the most common drug test. Why is it that the cocaine user that used their drug of choice a few days ago able to maintain employment while the marijuana user who used their drug of choice a few dayds ago fired?
Why is it that alcoholics are protected under the American's with Disabilities Act while illegal drug users are not? Isn't illegal use and especially addiction a health problem?