A little bit of history about why cannabis is illegal in the first place.
It all started with Harry J Anslinger, the FBI director during the 1930's. When the FBI was in danger of being defunded, he needed a new drug to scare the public with. He associated Mexicans (who were not well liked at the time) with cannabis (this is where the term marijuana comes from). He testified before congress saying that "the drug causes violence and death in its users". In 1937, the first anti cannabis laws were passed by congress. Making your first time getting caught with a single joint punishable by 2 to 10 years incarceration.
Fast forward to the Nixon era, there was a huge push by the American people to legalize cannabis, Nixon being Nixon vetoed it. One of his top advisers, a man named John Ehrlichman, said in 1994; "We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana, and the black with heroin, and by criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did".
Scientists knew cannabis didn't cause death or violence, it was proven in the 1940's. So, basically, the whole reason cannabis is illegal in the first place is due to a racist idea to gain money, and the reason it's still illegal is because a power hungry Nixon wanted to eliminate his political rivals and minorities.
I'm not making a word of this up. Cannabis was totally legal and fine to use in the US and around the world well and truly into the mid 1930's. It was a racist, expensive and jealous idea made by corrupt, power hungry, lying government officials.
Regardless of what people's opinions are on the legalization of the drug now, knowing that the only reason cannabis is illegal right now is because of racism and political rivalry, how can you defend its criminalization in the first place?