In 1937 the U.S. passed the "Marihuana Tax Act of 1937". 'The Act did not itself criminalize the possession or usage of hemp, marijuana or cannabis, but levied a tax equaling roughly one dollar on anyone who dealt commercially in cannabis, hemp or marijuana. It did, however, include penalty provisions and a complex Regulation 1 codifying the elaborate rules of enforcement marijuana cannabis or hemp handlers were subject to. Violation of these procedures could result in a fine of up to $2000 and five years imprisonment. The net effect was to make it too risky for anyone to deal in the substance until World War II required the United States Department of Agriculture to make its 1942 movie "Hemp for Victory".' - from the Wikipedia Article Marihuana Tax Act of 1937.
This act was later deemed unconstitional by the Supreme Court in 1969 during the case Leary v. United States because it violated the 5th Amendment since a person would have to incriminate him or herself to obtain the tax stamp required by law.
The government then passed the Controlled Substances Act (CSA) in 1970 and placed Marijuana on the List of Schedule I drugs. This means that it meets the following requirements:
(A) The drug or other substance has high potential for abuse.
(B) The drug or other substance has no currently accepted medical use in treatment in the United States.
(C) There is a lack of accepted safety for use of the drug or other substance under medical supervision.
If this were true then why is Marinol, a synthetic version of THC, the main active ingredient in Marijuana, only on the List of Schedule III drugs? To be on that list it meets these requirements:
(A) The drug or other substance has a potential for abuse less than the drugs or other substances in schedules I and II.
(B) The drug or other substance has a currently accepted medical use in treatment in the United States.
(C) Abuse of the drug or other substance may lead to moderate or low physical dependence or high psychological dependence.
This list also includes drugs like Tylenol 3 and Anabolic Steroids. So why is Marijuana illegal because of THC and Marinol is legal even though it's synthetic THC?
Change.org also notes that the government can tax the hell out of it, regulate it, use the production of it to create jobs, etc. It's an interesting debate, for sure.
If you are so inclined, sign the petition to legalize marijuana here.
Here's a cheesy clip for your viewing pleasure: