My lecturer had a discussion about this with us yesterday ironically and he brang up most of the points that TheVirus did. Hemp is a natural material and couldn't be trademarked by a company and produced by them (Hemp paper doesn't turn yellow over time). It also made people question war tactics and gave them a mellow attitude to fighting a war, thus it was banned and anyone found smoking it was sent to prison.
The US alone has a $5 Billion budget to tackle drugs in it's homeland, 4 billion purely for marijuana and $1B for crack, cocaine and any other hard hiting drug out there. I think the figure is something like 80% of prisoners are inside due to marijuana offences. He also said that the prison industry is the second biggest in america revenue wise (not sure if that's true).
Makes you think about it? ...right