A reader hits the nail on the head:
The difficulty with marijuana is that it produces a side effect that our government cannot tolerate. This side effect is so severe that any drug that produces it must be severely restricted or banned outright. And it is an insidious side effect. It is so insidious that it is nearly impossible to detect through measurments of body chemistry, metabolic function, critical organ functions, or tissue damage. You simply cannot find any harm caused by this side effect, but it's there.
The side effect, of course, is pleasure. Our government will never allow it.
This is the nub of the issue, I think. Sometimes, you hear attempts at justifying the ban on pot that point to marinol, a THC-based drug that allegedly helps nausea. They're for that, if necessary. And they much prefer it to marijuana, even though smoked or vaporized THC is much more effective. Why? Because marinol doesn't provide pleasure. And pleasure, even harmless pleasure, is evil and must be prevented. Once you allow people to enjoy life, there's no end to the dangers. Unless, of course, pleasure is backed up by vast industries rendering hefty taxes, like tobacco and alcohol. Then it's fine.
For my part, I find the attempt to ban any naturally growing plant to be an attack on reality, and a denial of some of the most basic freedoms. I guess that's why today's GOP is so in favor of it.