Well, you know, deep subject.

If the job has any moving machinery, risks of injuring others, risk of loosing appendages, risk of an incident that will really screw things up...well, ya, I 'spect an employer is correct in that concern.

Conversely, if it is an office job putting mail in a slot or some safe thing like that, I see no issue. Not a bunch of jobs out there like that. Lets face it, it does carry risk of doing something stupid, especially for folks that just get high. I would sure as hell worry more about the drinkers on the job than the pot heads on the job.