You can drive with heroin in your system if you have a prescription for morphine. Even a co-codamol headache tablet would make you positive for heroin.
Now be honest have you ever taken a cocodamol for a headache and drove? Were you anymore dangerous than normal?
The problem is “testing positive” and being “under the influence” are two completely different things.
With the legal levels set so low you test positive for a joint you smoked 4 days ago.