Well, again, there's a safety issue with that. "Pot"'s intoxication lasts into on-duty time and affects performance. It is not unreasonable to require employees to be sober while on duty, especially when one's dealing with dangerous heavy equipment.
Or pushing a little red button on a nuclear submarine. ;)
there's a big safety issue with drinking alcohol or beer too. why not fire people who drink beer on the weekends, too?
a good drunk can often last into working hours, and a bad hangover almost certainly does.
That being said, I once worked with a guy (CAD draftsman) who always seemed to be in slow motion and somewhat 'out of it'. He was subsequently let go basically for being slow and not worth the union scale he was pulling down ($60/hr cost to Co).
He went to another company and was put in the field at a power plant. Low and behold he failed the 1st drug test - along with his SON no less.
To me that answered why he was always in slow motion and 'out of it' - his brain was half fried.