Here's [1] a good rundown of the conspiracy. To wit, it's a conspiracy of two: DuPont wanting to outlaw hemp which could have provided an alternative to Nylon, and Hearst which was heavily invested in wood-pulping machinery for paper production, again for which hemp was a good alternative.
I have trouble with that article due to the tone. There’s no reason to refer to people as ‘potheads’ or mention Jerry Garcia with bitterness and disdain, or at all. It seems the author has an axe to grind with the remnants of 60s culture.
1. https://www.alternet.org/story/77339/debunking_the_hemp_cons...