> And addicts are addicts for life
Ethics aside, that is the problematic part of your reasoning. First, only less than half of treated patients relapse [1], second: a relapse is not a measure of a failed treatment as it is a qualitative value (relapse happened/didn't happen), not a quantitative one (to what degree did the patient relapse into old behaviour).
More importantly, it most probably costs more to incarcerate patients than to treat them. The US has the highest incarceration rate in the world. Thats expensive: $47.000 per inmate per year. [2]
[1] = https://www.drugabuse.gov/publications/principles-drug-addic...