Public approval of drugs changes rapidly with time and it doesn't quite happen with the wisdom of some supernatural higher power. It's actually a quite natural process of propaganda for profit, or an entirely divided action from the drug itself aimed at a certain group from a higher class.
Take opiates for example. In the 19th century they were legally grown in the U.S. and used as painkillers in the Civil War. When the Chinese came over to work on the railroads bringing a little opium with them, the white laborers felt threatened by their cheap labor, pushing for drug restricting laws intended for the Chinese-not the opium- making it illegal. Since then, the Bureau of Narcotics has launched thousands of campaigns to persuade the public that drug users are dirty criminals.
People once looked upon with a medical problem were immediately labeled as scandalous. This also sounds a little similar to the Marijuana Tax Act, which was stimulated because the Narcotic's Bureau budget was cut by Congress-not because we were all so concerned about the moral decisions of children.
It's as if an evolution of mind-altering substances have repeatedly represented the motif of an era in a silly trial-and-error landing. One minute alcohol is completely prohibited and the next we're pouring it all over our bodies in music videos. Marijuana is only for poor little minorities, talked about in whispers and suddenly it's the most treasured item during a 20-year American hippie fest and then suddenly shunned again. Where is it all going to fall into place next?
Once a drug shifts from lower to upper class, it's passed around a little bit more in the open. When marijuana shifted from poor Mexicans to white college kids in the 60's, concerned parents didn't really feel like having their kids being stigmatized, so many of the rich and powerful whites simply used their influence to liberalize drug laws.
Yet, before that, with the help of some slick propaganda, marijuana in the 40's was portrayed as a drug of crime and promiscuity and it wasn't until the Vietnam period where it became questioned again and widely used. Although it remains false that marijuana is physically addictive and a gateway drug, there's the majority that still strictly fear and reject it and rejoice in these stereotypes.
What about the one drug though that kills more than homicide, alcohol, cocaine, heroin, fires, suicide, aids and car accidents combined? For some odd reason, this one's legalized above many others. With the aid of millions of dollars to political campaigns and massive investments in advertising, the government refuses to make nicotine illegal because tobacco obviously makes a lot of money and pushes our economy (with the sacrifice of a few half a million citizens randomly dropping dead from it each year.), apparently there is no economic use in marijuana.
Though most of us are too busy to sit around and ponder the meaning of our socially constructed laws, there are some people who aren't. The rich interest groups that enforce these aren't always enforcing them to make our lives a little more comfortable, but theirs, and the abundance of criminals, mostly incarcerated for drugs, have been created from an abundance of criminal laws.