I think RED, that it is not the principle of drug testing in theory, or an assault on you.
I am sure that you are a very decent and well intentioned person.
I also believe that rights exist in case, not because.
If you and too many others come to believe that exercising your rights is only for instances where YOU PERSONALLY will benefit, than they will slowly but surely slip away.
I do not intend to insult your intelligence, but I strongly encourage you to read more history. People gave up their rights in exchange for stuff.
That stuff was safety (which you advocate), because it did not affect them personally, (which you also advocated) and because they were promised financial gain. (not sure where you are on that one.)
My point is, little by little, our rights are being eroded, and when it reaches a tipping point, there is no redress of government. The government has total control. We need to exercise our rights to keep the government, and everyone else honest.
Your example of the postal employee, a federal employee, is a sad one. It really shows how the federal government views its "subjects". Do I need to remind you that we are a government of for and by the people?
In my view, if you are trusting with people, they will be trustworthy.
People can rise (and fall) to the level of expectation applied to them.
Also, regarding a comment in which you mentioned customer satisfaction,
I would assume that if an employee was so stoned that it left a bad impression on a customer, you as EMPLOYER might see that as well, and deal with them accordingly.
I am not trying to be an advocate for those that would abuse drugs on the job, nor am I saying that you shouldn't have the right to run your own business as you see fit.
Its just that in my opinion, if you required a drug screening pre-employment, and no one showed up, you might have to revise your position.
Unfortunately, more and more people have been forced due to economic circumstances, to cave in, and pi$$ away their civil rights.