Assignment:
I think one reason the shift is toward a rehabilitative role versus a punishment role is that some people feel that drug addicts are more likely to come forward and ask for help if there is no chance of them being punished or incarcerated. Drug addiction is a slippery slope, one that can potentially set people up for failure. If they catch a charge for possession and are placed on probation, then test positive on a drug screen during probation, their probation is violated. In most cases, at least in my jurisdiction, probation violations are punished pretty firmly. This can create a vicious cycle of incarceration and many people feel this type of punishment isn't the right answer.
The conversation about drug addiction and how to resolve it is one that is heavily debated. Some people feel that addicts are safer in jail because at least they can't use drugs and potentially overdose, but others feel that incarceration isn't the answer because they aren't treating the problem and when they're released, the problem is still there and they go right back to the drugs. There are drug addiction programs offered in a lot of jails/prisons now, which is sort of a compromise to both arguments. There is still an element of punishment, but there is also an element of rehabilitation.
I honestly don't know that there is a right answer to this debate because everyone is different - we can't solve everyone's drug addiction the same way. I believe there is a time and place for punishment and a time and place for rehabilitation. I think that each incident should be determined on a case-by-case basis depending on the elements of the crime, the criminal history of the person, the person's drug-use history, etc.. Depending on these details, the best course of action can be determined for each individual person.