While I hate intrusion into privacy, I do think a employer should have the right to require drug tests if he wants to. Not because he should care what a person does on their own time, but rather because it could be an indicator of an up coming problem, an explaination of unsual behavior or an easier way to rid themself of a liability. I have known of more then one blue collar worker that started embezzling and clost their companies dearly. One guy I know who had his own company worked up to a coke habit of over a $1000.00 a day (I thnk he said he was up to $10,000 a day at the very end) before he collasped his own company into bankruptcy and lost everything. In that case it only hurt himself and his family, but amazing he had so much energy he was doing twice as much work as before. Until the very end, no one knew what was going on.
There is also a question of liability involved. What would happen if an employee comes to work stoned then gets hurt on the job. What if he delivers a package and hits and kills someone. I never have had enough employees to justify it, but I have had an employee embezzle from me and another (who drove a company vehicle) relapse on a bad cocaine habit and end back up on the street. I got lucky in that I wasn't harmed financially much, but either situation could have ended up ruining my company.