The first significant instance of cannabis regulation appeared in District of Columbia in 1906.Regulations of marijuana (the phrase Indian Hemp is sometimes used) followed in Massachusetts in 1911; Maine, California, Texas, Wyoming and Indiana in 1913; New York City in 1914; Utah and Vermont in 1915; Colorado and Nevada in 1917. These laws were passed not due to any widespread use or concern about cannabis, but as regulatory initiatives to discourage future use. http://en.wikipedia.org/wiki/Legal_history_of_cannabis_in_the_United_States "Make the most you can of the Indian hemp seed. Sow it everywhere." -- George Washington, in a letter to his farm manager Thomas Jefferson, the co-author of the American Declaration of Independence was the third president of the United States. He cultivated cannabis. Benjamin Franklin, shown here on a U.S. $100 bill, started the first American paper mill, which made paper exclusively from cannabis.
Not sure if this was mentioned yet, but I know one of the arguments against legalization and regulation I have heard is that THC levels are a lot harder to test/regulate than say, alcohol levels. One of the only reasonable arguments I've heard.