I think marijuana was first looked down upon because of the assumption that 'recreational = bad'. The criminalisation and war on drugs simply made people turn to shady methods to obtain it, making it and its users look even worse in the public eye. When evidence pointed to the contrary, some people didn't want to change their existing assumptions and continued under the 'gateway drug' guise. "That's the way it's always been done" and whatnot. Conspirators argue that corruption and conflict of interest in court plays a part in this. Judge has stakes in private prison, convicts people with possession of marijuana for ridiculous number of years, gets higher value in prison stakes. For the most parts, I'd say politicians really do believe in many of the sides they fight for. It's politics and you know that if you don't play dirty, your political enemy will and destroy you in a heartbeat.