As some above mentioned, I believe, as far as Cannabis is concerned, that it all boils down to level of impairment. There is, as yet, no standard for pot users, and that's really the problem, though the more states that legalize it, I think that will soon have to be determined. But as cannabis-tolerant I am, I don't care to be put in danger by any drunks OR stoners.
BAC has a standard of impairment, and even though some drinkers may indeed be unaffected by certain levels they are still subject to that standard and being fired or arrested. Some jobs have stricter BAC standards as well.
But alcohol leaves the body rather quickly, unlike cannabis that can be found in hair follicles and such for quite a while. But where will the impairment level be measured? Urine? Hair? Blood? Is someone impaired if cannabis found in their hair? I doubt that but I'm not the expert. Until a medium of measurement is determined and a standard is set I think it'll be business as usual - testing positive by urine or hair, whatever, is going to mean trouble for the person.
I know in the airline I worked for before retiring, if you damaged anything, planes, any equipment, any 'incidents' with collateral damage or bodily harm, it meant instant drug test. If you refused, you were gone, and if it returned positive, you were a goner. Plus, some jobs were additionally subject to randoms, and some folks were caught that way.