What is their rationale? We have alcohol and nobody believes that should be illegal. If one is for freedom and liberty one is supportive of legalization.
I did a dissertation-type study on Marijuana at Oregon State in Corvallis back in the early 1980s. The science of the then illegal drug cited numerous cases of families in Central America giving their children Marajuana cigarettes to pacify them, and the boys who took them had a failure of their reproductive organs where their testicles did not descend, and after too long, they became both impotent and unable to reproduce when they became adults. The other problem the disease sticks developed for all children who were put on the substance early in life was failure of brain development to the point they had impaired reasoning skills, some to the point of lifelong mental incapacity and disability to care for themselves as adults. The term "Deadheads" became pervasive in the American cult who took pot when it was illegal to erase their sadness (which it didn't longterm) and make them feel good (lessened by serious stealing when the drug became too expensive on the street.) It was considered the gateway drug to harsher addictions, irresponsibility, and incarceration for thefts, since all their money "had to" go for their addiction to the substance, and working for a living was not an option due to perils of the drug reducing concentration in jobs where that is a liability for the person and others.
Marijuana ws and still is hell for drug abusers, because today's marijuana is exponentially more deleterious than its weaker predecessor back when and before growers figured out how to increase its potency. We can thank Hollyweird for making it a stupidly revered social happening. If you're a guy, Take marijuana. Your testicles will not descend in puberty if you start young enough.

And you, too, can sing "Don't know much about history, don't know much trigonometry...." etc.