Hemp was the major staple of George Washington's garden. Hemp was sold in American pharmacies. Hemp was required to be grown by colonial farmers. So why, now that it's illegal, is it referred to as marijuana? The term marijuana has a short history in America, filled with racism and political intrigue.
Although the middle east has a history of hashish use, hemp wasn't really smoked recreationally in America until Mexican immigrants brought the practice with them into the western states around the 1930s. California was the first to ban the "preparation" of hemp, after Mexicans were observed doing it, and many western states followed. This was happening right at the time of the prohibition of alcohol, and the government was testing how far it could go banning mind-altering substances. Apparently, it found that it could go quite far.
Rumors began circulating linking marijuana use to murder and violent behavior, but it was obviously unfounded speculation based on the fear of other races. For example, newspaper tycoon William Randoph Hearst writes, "
As soon as hemp was in non-white hands, it was labeled with the exotic term "marijuana," which the layman did not know was the same plant. William C. Woodward, of the American Medical Association, cited this as a reason to reject the bill proposing its prohibition. In the article I linked to above, the author writes,