In addition, our current system is kind of a hybrid of the criminal justice and public health approaches to addiction. A lot of addicts get treatment because they have been mandated through the legal system. Drug courts are empowered to impose treatment as an alternative to jail, addicts who have been busted get treatment to lighten their sentences and parents seek help for kids early on because their kids got mixed up with the law.
There are valid theories that support legalizing drugs: it might eliminate criminal activities around the illicit drug trade, it would reduce crime, it would transform drug addiction from a criminal issue to a public health issue, clean needles would reduce disease etc. In support of those theories people often point to real world examples such as Switzerland and Portugal (see for example The New Yorker article, October 19 issue, about Portugal). But those situations have yielded mixed results, the data is not complete and a lot of subjective interpretation of the data is going on.
I am not saying that avoiding legal consequences would be sufficient motivation for anyone to remain clean (although it might be) nor am I saying that there aren't other motivators aside from legal to seek treatment. I am just saying that the legal system appears to have play a positive role.
PS. The Supreme Court in a landmark and controversial case decided, based upon the doctrine of separation of church and state, that state supported drug rehab programs (whether prison based or via parole or probation conditions or through hospital or clinic rehabs) may NOT require attendance at Alcoholics Anonymous or Narcotics Anonymous programs because of references that the 12 steps of AA make to a "higher power". This despite many interpretations that a higher power is not necessarily a religious concept - many in AA interpret God to be "Good Orderly Direction" or "Group Of Drunks" (referring to the AA meeting itself) and despite significant evidence that AA and NA are by far the most effective long term drug treatment programs in existence. As you can guess, I strongly disagree with the Supreme Court decision.