Alcohol test simply shows it is in your system and what quantity, not how it affects the individual.
But there's predefined limits to what is considered to impair your ability to perform complex functions. Having the equivalent blood alcohol level of having consumed 100mls of 4% lager will not get you fired from work, because it's seen as being well below the limit at which alcohol will affect your performance.
The same thing doesn't apply to drugs - any sort of positive result and you're out, whether it affects your performance in your role or not. There's no comparable logic between the two processes, especially considering how much more detrimental to your state of mind is alcohol than many illegal drugs, and the distinction comes strictly from that definition - alcohol is a
legal drug, that is to say, taxable, and so it's portrayed as being more socially acceptable.
Given the modern hysteria over anything the troglodyte-pleasing tabloid tripe condemns in the name of selling their rags to mouth breathers without the mental capacity to know better, do you really think that alcohol would be legalised and as widely-spread as it is now, were it discovered today? Extremely addictive, devastating to your personal health, wildly psychoactive and more prone to driving people toward violence and beligerence or black depression than anything else. I can't think of many other drugs more deserving of being Class A than alcohol.
Edit:
You have no idea how it will affect you or how long it will last. You also can not prove when you took it.
That's not a response, it's just spamming the same tired, baseless argument that was just refuted.
Ah so people over the drink limit could and probably still are fine, but that's fine. Evedience shows that taking any number of drugs causes problems and companies have to protect themself and there employees.
Still not quite on the money, there. We're not talking about how close people can get to the limits while still being fine, but more the existence of such limits for "accceptable" narcotics but not for untaxed substances, often much, much less potent.