If I remember correctly, both Monsanto and DuPont wanted a ban on cannabis because they fear legal growing of a related plant, hemp, would cut into sales of synthetic fibers. But Dow Chemical and Johns-Manville put their foot down and said no (especially Johns-Manville, since they discovered hemp fibers made for a great house insulating material).
And thank G*d it stayed legal--in the countries where cannabis was banned, we had a large spate of people dying from people smoking cannabis loaded with sometime quite dangerous substances. Keeping it legal meant the FDA and BATF could strictly enforce purity standards, and we didn't have a spate of people suddenly getting seriously ill or dropping dead from smoking one "joint."
Besides, with hemp growing still legal, it made it possible to build lighter airplanes--General Aviation airplanes built with hemp fiber structures by the late 1950's were very popular, and General Motors--who had experimented around with fiberglass structures on the Corvette prototypes in the early 1950's, ended up with hemp fiber structures for that famous sports car.
Hemp fibers are still relevant even now, thanks to it being part of the composite structure used on the Boeing 787 (OOC: this was probably the name of the shelved 7J7 project to replace the 727 and larger 737 models from the late 1980's powered by two GE UDF propfan engines) short to medium range airliner, the plane that has set new standards for quietness and fuel efficiency even now.