It's not about whether they can or can't. It's about the massive liability for the company if they fuck up on the job and cause property damage or bodily injury.
That could be true in a causative sense, but not a correlative* one. Not that it's ethical to do so, but by testing for marijuana smokers, businesses/insurance companies may by proxy be testing for other traits likely to cause accidents
Can someone who smoked a joint a couple days ago be trusted not to smoke one 30 minutes before starting work with the concrete cutters? Trusted enough to bet your company on? (Because you know that if the worker smokes a joint just before working with the concrete cutters, and someone gets injured or killed, the owner loses the company in the lawsuit.)
> Can someone who drank a beer a couple days ago be trusted not to drink one 30 minutes before starting work with the concrete cutters? Trusted enough to bet your company on? (Because you know that if the worker drinks a beer just before working with the concrete cutters, and someone gets injured or killed, the owner loses the company in the lawsuit.)
But I imagine if we interviewed 20 people for a position, we would most certainly not have 18 no-shows.