Couple of points:
1. As Alex points out, the test covers a whole raft of forbidden compounds: THC, opiates, cocaine and derivatives, methamphetamines, PCP, MDMA ("Ecstasy"), and bog knows what else*, so even if a medical card was a "Get out of jail free" card for the first one (it certainly isn't), they'd justify it on the basis of the others.
2. I don't have such a card, nor do I suffer from any conditions that might realistically justify one being issued to me. Of course, this is true of a number of people I know who are currently being prescribed magic cookies.
3. Given what we have learned about the levels of surveillance to which we are subject domestically, and the overall trend toward much more of the same in the years to come, it seems not unrealistic to imagine that should I be granted such an instrument, the day is not far off when some unhappy confluence of interlocking databases spits out my name in this connection together with BDS, and thence to DHS, which is a strong believer in its affiliates public and private maintaining a "drug-free workplace."
4. As it happens, the only thing the piss workout is likely to yield would be aspirin and statins. I've long found cannabis to be hard on the windpipe and personally impractical as a social lubricant, so although most of my friends between the ages of fifty and seventy still indulge on occasion, it is my custom not to join them. The knowledge that this virtuous abstinence is now, so to say, enforced rankles quite a bit.
*5. As to "who knows what else," it's amusing to consider that I could have done a dose of LSD the night before, and still awaited the test results with a placid disposition, since the effective dosages are measured in millionths of a gram, and these are largely metabolized during the fraction of an hour before the onset of effects. Apparently trace amounts remain in the liver for a few hours, but it would require an actual biopsy to access the affected tissue, and the day that becomes a condition of employment will be the day I head to HR and turn in my building pass. I hope. Anyway, who knows where to find ergot-derived hallucinogenics anymore?