While there is a compelling reason for some to have mandatory drug tests (and appropriate follow-ups should the initial test turn up positive) for those who are on the forefront of public safety (truck drivers, pilots, etc.), drugs tests without a probable cause or sufficient protections against false positives are morally wrong in my opinion.
The event that initiates the test in these situations is the simple fact that a person wants to or is holding down a job. That's not normally the modus operandi of a drug addict. Furthermore, one can easily get false positives from consuming things like poppy-seed rolls, going to a concert or other public event where people are smoking cannabis (I once walked out of a White House tour directly into a crowd of people smoking pot as part of a protest for the legalization of cannabis... I didn't have a choice about it, the Secret Service directed us out that way), or simply the use of legal prescription drugs that you may not recall before taking the test. There's also the issue of the possible mistakes/mislabeling in the lab that is processing urine. That job is not necessarily done by people with advanced degrees (or even a degree) or folks who are particularly responsible.
Furthermore, having some near family members who have had severe drug problems, many drug users know how to cheat the tests and can name the appropriate prescription drugs they are allegedly taking in order to mask the fact that they're actually taking illegal drugs. As a result, drug tests are much less effective than many in the law-abiding world believe they are.
Often, with mandatory drug tests, any positive is an automatic verdict of guilty, without any follow-up or recourse.
As someone who has never used an illegal drug, I'm irritated enough with the written tests that I've had to take in the past when seeking employment. Because I don't admit to marijuana use (since that's actually the truth), I have been denied for jobs from several employers. They assume I'm being dishonest since "everybody has done it at some point."
At a previous employer, the HR people wanted to institute a written test for all new applicants to a position in my department to save money during the hiring process. Since I was doing the hiring, I negotiated with HR that we not use the test if I failed it. They had known me long enough that they didn't have any concerns about my character. I failed the test spectacularly since I told the truth and did not admit to any illegal drug use which was expected for my demographic. When I was first hired for the entry-level position, they used a battery of personality and intelligence tests and a 30 minute interview with a psychotherapist to determine if there were any issues. She asked me about illegal drug use and I told her the truth. She believed me because, based upon the information she had about my personality, background and core beliefs, it made sense to her that my statements would be true.
Getting back to the original issue, there must be a compelling public safety issue for mandatory drug testing, along with an appropriate follow-up in the event of a positive result from the initial test.
Otherwise, there needs to be probable cause to initiate a drug test. Simply wanting a job is not probably cause.