When I was a dope-smoking student, there was a bit of a cult book going around which had recently been published called 'The Emperor Wears No Clothes' which claimed that cannabis was only outlawed in Western countries in the 1920s was because of lobbying by the oil industry - DuPont, in particular. Their interest in this was, allegedly, that the hemp plant is the most cost-efficient source of all sorts of raw materials, including cellulose, which can be used to create biodegradable plastics.
Knowing nothing about industrial chemistry, I have no idea whether this is all drug-addled conspiracy nonsense, but the book does at least provide sources to back up the claims and a quick internet search doesn't throw up the deluge of unfavourable fact-checks by the scientific community which you usually get with this sort of stuff. BS or not, it's still an interesting read:
https://upload.wikimedia.org/wikipedia/ ... lothes.pdf