I disagree.

The way I see it pot is illegal because hemp was a threat to lumber and textile industries.
In fact the term 'marijuana' was never used in the US prior to the early 1900s when a man named William Randolph Hearst, who happened to own the majority of the media, started sensationalizing stories of how mexican immigrants smoked this evil 'marijuana' and went on murder rampages. According to Harry Anslinger marijuana was much worse than cocaine and heroin and even caused African americans to become insulant and step on white mens shadows.
In short the Marijuana Tax Act of 1937 was passed to protect the interests of lumber giant Dupont and newspaper king Hearst. Incase you didn't know an acre of hemp can produce as much paper as four acres of trees and has thousands of other commerical uses. The Marijuana Tax Act was later found unconstitutional thanks to LSD guru Timothy Leary, in response congress passed the Controlled Substances Act 1969.
Read all about it at www.jackherer.com/chapters.html

By the way, rumour has it opium was made illegal to spite chinese immigrants and I wouldn't be surprised considering the horrible treatment they recieved here in America.