OK, I would foresee the following:
As mentioned, many workplaces would maintain testing for safety reasons, and to that you’d add those who may not test preventively but would reserve the right to test “for cause”, when there is any suspicion that you have been working while impaired – in order to make things fair, this would also have to apply to alcohol, of course.
The terms would probably have to be something clearly spelled out at the time of hiring and before you accept an offer of employment. One thing that would further level the playing field would be if the technology were developed to measure for various nonalcoholic intoxicants, with roughly the accuracy of the breathalyzer, if you are actually under the influence NOW, as opposed to having traces of heving been exposed some time in the recent past. After all, right now if you’re a machinery operator and have an accident, the Man would really like to know your BAC at the time of the accident, not what you drank last Saturday night. This is of course a bit more complicated then, the various psychoactives run the gamut from mild sedatives to strong narcotics, from moderate euphorics to violent stimulants (and do I really want my salesclerk to be alarmed that if she opens the cash drawer, army ants will crawl out and climb up her legs?).
This would probably mean that unless you explicitly contracted otherwise with the employer, then they would have to prove that you were impaired while on the job, and there would have to be no penalty for partaking in your off-time, any different than if you drink on your own time. But there’s the rub. What keeps a private employer from saying we will not contract for employment with a person engaging in certain conducts? In many “employment at will” jurisdictions, on those issues that Equal Employment/Civil Rights laws don’t explicitly protect, your choices are jumping through the employer’s hoops or applying elsewhere.
And that brings us back to the “undesirables” issue based on cultural/social valuations: that an employer may be basing his hiring or promotion policies on “I don’t like the kind of people who smoke pot and munch granola; I like the kind of people that knock back beers and grill steaks”. Or even, “Goodness gracious I can’t stand those twitchy, googly-eyed, hyper speed freaks – give me some nice, mellow, sunshiny granola munching potheads for the office”. Without true sociocultural change (say, it were a Supreme Court decision), it could be a mess.
Someone meanwhile would almost certainly argue that for reasons of saving on health insurance costs, this company will want to delimit certain categories of legal intoxicant and levels of consumption thereof that we consider specially risky, and monitor you for it. This, however, creates a problem in that the insurance company would probably have more to save by getting quite intrusive re: consumption of tobacco, alcohol, trans-fats, and other legal “risks” … and the most that would happen is the insurance rate goes up and it gets distributed among the entire payroll.