[Spectato]

I think the primary justification behind this (and Methadone programs generally) is, that it's preferable to have addicts get their fix 'safely' from the state, rather than need to go out and mug people's mothers for drug money three times a day.
Which also subsequently benefits the drug cartels etc.
That and the fact that methadone is a nice little earner for those involved in it's dispensation.

(Google: methadone profit)