We are simply tested for the use of certain drugs, not for impairment.
Alcohol is legal to use and quickly eliminated from the body. Low amounts of alcohol are not a problem, although it's very easy to go over those low limits.
Cannabis, on the other hand, usually is illegal to use, even in places where it's been decriminalized, plus it's slowly eliminated from the body, so that the detection of trace amounts even a month or more after use, when there's almost no question of impairment, can be as bad or worse than having a too-high BAC.
Some of us may remember that scene from
Pulp Fiction where Vincent Vega is telling Jules about the "hash bars" in Amsterdam. Check this out:
"Cannabis is less harmful to a user’s health than hard drugs such as ecstasy and cocaine, but it remains an illegal substance. This means that the trade in as well as the sale, production and possession of this drug is punishable by law. A tolerance policy is active in the Netherlands which means the possession and sale of soft drugs is recognised as a violation of the law, but isn’t prosecuted." (
FAQ Coffeeshops in Amsterdam | I amsterdam)