I don't find pre-employment drug testing terribly problematic, although I would be fine seeing it discontinued in most settings. While I don't use drugs myself I don't have any ethical problems with drug use and I'm frequently in the company of those who do use drugs.
Every single one of my friends smokes weed regularly and are employed despite most of them being subject to drug testing during the hiring process. To me, all a negative drug test shows is that a prospective employee either doesn't use drugs(which is probably a relative rarity these days), or that they do use drugs but are either resourceful enough to find a way to beat the test or disciplined enough to plan ahead. I guess in this sense perhaps the tests do serve some purpose.
My only issue would be with random unscheduled drug testing which to me serves little purpose, outside of jobs were impairment may pose a risk to others.
Far more concerning to me are criminal records or poor credit being used to withhold employment from a large segment of our population.