it's true enough that drug addiction should not be punishable by jail time...nor should possession of drugs, in and of itself ("intent to distribute" is hard to prove). Habits are hard to break in prison and I hear tell getting access to narcotics is way easier in prison than it was on the streets...so rehabilitation in this sense is not an option once an addict is locked up...
don't even get me started on all those people locked up for marijuana possession...