Some of the is actually stems back to the war on drugs. The history of the war on drugs is about profits. Marijuana was originally made illegal through propaganda campaigns and it was actually in part to get rid of hemp which was huge competition to the paper manufacturing companies.
It is pretty well known now that marijuana in many ways is safer than alcohol as you can't overdose on it or get liver issues as far as is known about it. It also doesn't cause physical dependence like alcohol does.
As to parasites and the Government I'm not sure but I do think many doctors are legitimately ignorant. They are stuck in the box of what they learned at medical school. People from the forums here know more than most these doctors about parasites. It's messed up but that's how it is. Ignorance, apathy and laziness. Maybe it does go deeper though. Why are there no big studies on the link between cancer and parasites?