Lets face it, its much less harmful than alcohol and will eventually be legalised, but for now its a big driver for bitcoin.
But the cannabis is legalized in the US now, why would anybody buy from the darknet?
Simple, its because it is still illegal in some parts of the world so they are purchasing from darknet markets "anonymously". I guess that is much safer for them to buy online than in real life (probably because of the risks having a deal to an undercover)
I read some where saying that the silk road market reduced some kind of drug related crimes a lot. Because people did not have to trade face to face.