This is like saying, we shouldn't arrest drunk drivers if they aren't involved in any car accidents. It's the whole principle of the possibility of abuse. There must be some sort of punishment. And you can't base this off the assumption that people will always be drawn back to their illegal substance.
Jails and prisons torture people with impunity. All that torture over mere possession/sale/purchase/consumption of intoxicating substances makes it MORE likely the person will remain intoxicated when his/her access to the intoxicating substance resumes and possibly commit violent offenses while intoxicated. Modern psychology knows better than society just why incarceration is ALWAYS INEFFECTIVE.
As silent z has stated "its the whole principle of possibility and abuse" and to that I respond, there is a huge difference between drug use and drug abuse, but I digress. My point is that just because somebody does something considered illegal does not mean they should be put the same place as actual criminals. Being incarnated is NOT going to teach non-violent drug users anything besides being better at committing crimes. Think of it as a finishing school for criminals.