You always have the choice of declining employment by anyone who wants to test you.
Yes, as I would (which I thought was basically what the OP was asking-who would not consent to this).
Is there really no job you can think of that you think should be able to require drug testing? Would you hire a full time live-in nanny if you knew she was a heavy heroin user but currently appeared to be a functional addict? Would you vote for a President known to be self-medicating his crippling depression with illegal opiates? Should Barry Bonds be allowed in the Hall of Fame?
There is no job that I can think of that I would want to have where I could imagine drug testing being justified. If I knew a nanny were a heavy heroin user, then no, I would not hire her. But, clearly there was enough evidence to "know" without testing her in this case, so what would the point of a piss test be? I would not insist that someone else who has given no signs of issues have their privacy violated in the name of my paranoia (and if there are signs of issues, I wouldn't hire, drug test or not).
And I'm not sure what level of drug use under what circumstances would influence my vote for the President and by how much, it would all depend. But, again, this would only be influenced by drug issues that are known b/c their are some other indications of them (although like I said, how much they'd even matter then would depend). I wouldn't support urine drug tests of the President.
And I don't know who Barry Bonds is, nor do I care at all about whether or not he's in some hall of fame, regardless he uses or has used drugs (which I'm assuming is why he was mentioned).
In some jobs (like spies) the no-drugs policy has nothing to do with potential on-the-job performance, and everything to do with your perceived security risk. Are you the kind of person who is subject to blackmail? Could a foreign intelligence operative potentially use your drug habit to influence your judgment, or convince you to divulge secret information? Drug use is still seen in some circles as a proxy for character flaws, at least in circles where "character" means blind obedience to authority and not rocking the boat.
I disagree with it indicating anything about "character," but as far as perceived security risk, I assume that if one has such a drug problem that they could be blackmailed about it, it will show up through other crazy-ass behavior, finances, and other things that a high level security clearance would show. If not, I question our security clearance processes, and adding a piss test certainly isn't going to fix them.