>Fire someone for showing up to work drunk, sure. But would you fire them for a bottle of wine on a Saturday night?
If alcohol was like THC (that is, it's impossible to determine whether you drank in the last hour or the last week), I sure as hell don't want people who test positive driving.
Still too broad a range, smoking some weed at 7pm doesn't necessarily make you unsafe at 7am. Zero tolerance is unrealistic and unnecessary for most jobs that drug test.
There's not another viable test though that can act on a shorter window than that. Companies aren't out of line for not wanting their employees to be working while under the influence of marijuana - the problem isn't that they're being too strict ('zero tolerance'), it's that they don't have a granular test that would suffice.
That, plus marijuana is still illegal in most states. Whatever your personal conviction may be, businesses aren't in the business to turn the blind eye to illegal substance use. They're on the hook for liability.
Oral tests are very inaccurate and the false positive rate is huge (can be more that 50% for some manufacturers, you're better off flipping a coin). To my best knowledge, oral tests are usually confirmed with urine and/or blood samples (at least that's the procedure if you test positive while driving here).
So as far as I know, there isn't a reliable and accurate way of testing.
Using DUIs as an example, these cover things besides alcohol, can be prescription drugs, probably other things that impair motor function (IANAL).
Roadside DUI test is a general test for motor impairment. BAC tests are a nice proxy for that in the case of alcohol, and a legally convenient one as they are precise and objective, but the police care (morally speaking) about your motor impairment not your BAC.
tl;dr, we can already answer the question "is this person too stoned to drive?", roadside DUI test.
Disagree. Motor impairment is one issue, yes, but so is mental impairment. People under the influence of drugs can have poor decision making, even if their motor skills are fine. Driving on meth is still driving under the influence, and those people would generally not have any trouble with a roadside gymnastics demonstration.
Fire someone for showing up to work drunk, sure. But would you fire them for a bottle of wine on a Saturday night?