It's not so much as punishing people for weed but punishing them for knowingly breaking the law right? Maybe locking people up is a bit harsh but people know the consequences no? Maybe we should recognize that there is value in having people in a society follow the rule of law.
How valuable to society are people who pretend not to be gay when it's illegal?
How much value was there in keeping black people out of the same restaurants as white people?
What value do you perceive society gains by locking people up for smoking a plant that makes you laugh?
On the contrary does it not remove value from society to stringently pursue punitive measures against something like being in possession of a gram of cannabis while higher crimes often face far less significant (relatively) consequences?