And do you think that drunks are any smarter?
The use of alcohol has far more profound and lasting effects, not only on the brain but also the whole body. It can destroy lives and would never make it past the FDA if it were discovered today. And yet I don't see anyone advocating alcohol prohibition.
"Potheads", like drunks, are the exception not the norm of users.
Potheas are the exception, just as alcoholics are the exception to people who drink