I read your post. If people can't stop their usage for a job it's more than likely an addiction. From my experience, I don't want to work next to anyone who is high or drunk on any substances. I'm ok with cannabis, so if workers want to smoke weed then do it after work and not before the shift. Most jobs that are actually worth a damn need the attention to detail. All substances will have an effect on your nervous system. Drugs alter reaction times, computation skills and overall health in the long term(depending on the substances.)
If I owned a business I would only hire A+ employees that I know would excel in their work. If they smoke weed or do psychedelics, cool, do it away from work and not on my dime. I've worked with ammunition manufacturing, real hard wood flooring, and now the cannabis industry. And from what I've learned, it's not the best idea to be high at any of those jobs. It's easy to mess up and it could cost the companies a lot of money. So in my opinion it should always be up to the employer on their drug policy. Not everyone deserves to be in the higher wage positions.
Sober workers have a better chance at going above and beyond what is expected. High workers are usually procrastinators. Not all, but most.