There is some truth to both sides. First, there is a difference between "jail" and "prison". Jail most commonly refers to a county, city, municipal lockup. This

There is some truth to both sides. First, there is a difference between "jail" and "prison". Jail most commonly refers to a county, city, municipal lockup. This is an individual's first stop on their way to prison (assuming they are found guilty). When you are first arrested you are taken to jail. At this point in the criminal justice system, there is no proof of "innocence" or "guilt". To be arrested the officer only needs to have probable cause. In many cities, counties, states simply possessing a joint is enough to get your arrested and taken to jail. Once in jail the problem begins, and this is a huge issue in criminal justice reform. Many of those arrested for simple possession of joint are poor and can not afford a cash bail system. This leaves them stranded in jail until their court dates which could be weeks or months down the road. Many jails face overcrowding by these individuals that have not been convicted of crimes but are simply awaiting their day in court. So while you are correct, most people don't go to prison for smoking a joint, many do go to jail for smoking a joint. They then sit in jail awaiting their day in court to plead their case and receive their sentence which is often no more than a monetary fine.