"Reefer Madness"

That was a very interesting video, @Richard Paradon , and although it is deftinitely out-dated, and highly over-dramatized; it certainly goes a long ways to see how the hemp scare in America came about.
Interestingly enough, growing hemp was a main crop in the United States, right from the beginning. One of the first "marijuana laws" that we had in America was a requirement for every farmer to grow hemp as part of their crops. It was used for making rope, clothing, paper, and many other important articles.
Our Declaration of Independance was written on hemp paper. Our very American flags were made of hemp, as well as the clothes that people wore, and it was much easier to grow than cotton is.
George Washington was a major producer of hemp.

From what I have read, pot did not become a recreational drug until fairly recently, time wise; although it was used in medicinal remedies. It looks to me like the use of psychoactive pot was cutting into the profits of big drugs, like heroin and morphine, and probably also affecting liquor sales; so they would have all been on the lookout for a way to criminalize the growing and use of hemp products.
Apparently, even growing hemp that does not get people high is just as illegal as growing the stuff that does do that. This doesn't really make much sense to me, since it could be used for so many useful products, as well as for the medicinal use of cannabis.
In many states, it still grows wild, left over from when hemp was grown everywhere as an industrial crop. It goes by the name of "ditchweed", and although you will only get a headache (and not get high) from smoking it, it is still just as illegal as the high THC varieties are if you are found to be growing it.

http://cannabisnowmagazine.com/cannabis/in-history/history-of-marijuana-in-america