I think the reason that doesn't work is simple. The US is already criticized for having the largest prison population in the world. And Officer Greg Hill, of the Eureka PD put it best on the TV Series Speeders. He pulled someone over for speeding. When the guy rolled down the window, pot smoke came billowing out. He could smell it, he knew full well what it was. He wrote the guy a speeding ticket, said please don't drive stoned and left it at that. The cameraman (or someone at least) asked him why he didn't arrest him. His reply was something like "If we arrested every pot smoker and put them in jail, where would we put violent criminals?" That is also why the War on Drugs has failed miserably.
He was just talking about the Humboldt County Jail, of course, and how there were only 417 beds. Humboldt County has a very low crime rate in comparison to some Counties of the same size. In 2014 someone murdered a Catholic Priest in Eureka. The suspect was arrested and taken to the county Jail. Where would they have taken him if the jail was full of minor drug offenders?
After as much traveling as I've done over the course of my life, specially with my band playing all across the country, we saw there are pot smokers everywhere we went. Pot may just be a plant, but the US government lists it as a Schedule 1 drug. The same as Heroin, Cocaine and many other dangerous, highly addictive drugs. And after personally being a pot smoker for 30 years, I laugh at the notion that it is addictive. When I stopped in 1999, I had no withdrawal symptoms at all. Unlike people withdrawing from Heroin. I just stopped and left it at that, never doing anything to do with it except make pot jokes since then. Some horrible case of drug addition, eh?
You've heard of MADD, right? Mothers Against Drunk Driving. We used to joke about HADD, Hippies Against Drunk Driving, for the same reason as Mothers, but also because Americans have been HADD by the government over false comparisons of the dangers of pot vs. Alcohol. Too much Alcohol is extremely dangerous, yet it's legal. Pot is much safer, and it's a Schedule 1 drug.