In many cases, the idea of rehabilitating people either before or after prison is nearly impossible. There are almost half a million people in the American prison system for drug offenses. If someone is released from prison for cannabis possession, they can likely drive a few hours to a place where it's perfectly legal (at the state level), and even encouraged by the culture. How could you possibly rehabilitate someone from something with those kinds of mixed messages?
The subject is analyzing prison's role in rehabilitation. If we consider prison a failure as a rehabilitation device, then maybe we should focus on rehabilitation before prison. This is a reasonable suggestion. However, if the reason they're in prison is for drug possession, which makes up a huge portion of the prison population, I'm suggesting that any kind of rehabilitation is nearly impossible. With cannabis legalization finished, many states actually are looking to legalize other substances, and even if they don't, there's strong social pressure in American culture to try other illegal substances, leading to a huge prison population.
So I'm saying that it in many cases, the problem isn't so much that prison is a failure as a rehabilitation center and we should focus on rehabilitation elsewhere, it's that people are in prison for things they simply don't want to be rehabilitated for and have strong influences to continue doing. The culture is inherently resistant to being rehabilitated from things that it doesn't consider harmful.
To use an example that might be more neutral, what if copyright enforcement was more severe and we had half a million people in prison for watching unlicensed YouTube videos? We might analyze the failure of the prison system in rehabilitating people from watching the wrong streams and focus more on education so that people could identify properly licened content before they watched it. But unless the people actually considered this behavior harmful, it would be an impossible task, and we would see ever increasing numbers of people in prison for watching the wrong videos.
But we also have very high recidivism rates for stuff like "armed robbery", independent of social-issue crimes like marijuana possession. And society generally has an Opinion about armed robbery. (It's bad!)
