Let them. Entire industries will avoid them. I work on oil drilling locations, and yes, I'd be able to pass. I don't drink either.
As far as off-duty use goes, alcohol levels diminish more rapidly than THC. So, while the "use" might be off-duty, the effects continue.
It is a safety issue, and the Government cannot force someone to hire anyone who doesn't pass the criteria for the job.
If they could, imagine the trainwreck (perhaps literally) which would come from that.
Weed is already such a low enforcement priority in NYC that you see people strolling down the street puffing blunts all the time. It’s already decriminalized for practical purposes so the “legalization” laws are just codifying the status quo. People who show up at work obviously stoned will get fired same as if you showed up after a 5 martini lunch—especially if you work in the kind of jobs you describe. The world goes on.