You're probably confusing decriminalization with legalization. Decriminalizing marijuana will not make it legal. Even if it was legalized though, that doesn't mean employers won't test for it. Some employers will not allow cigarette smokers to work for them, even if they smoke only when off work and not on company property. These polices have been tested in courts and the employers have won. If they can do this to people who smoke cigarettes, a legal product, they can do it to people who smoke pot even if pot is legalized.
“Because of the persistence of the substance, it makes casual use and employment mutually exclusive.”
Nonsense. Most pot smokers are employed. Look at the government statistics on this. The overwhelming majority are full time employed. Only a small percentage are unemployed. Plenty of casual pot smokers smoke a little weed in the evenings and/or on weekends and do a great job at work. You're just way off base here.
If I'm "way off base here," why do you need to send out a strawman to make your argument? That wasn't my point, and the rest of your post makes it clear that you knew that.