The idea that methadone is a "cure" for heroin addiction is beyond ridiculous. In many ways, it is more habit-forming then H itself.
For almost all the addicts I've ever hung around with, it was nothing but a stop-gap to be used when your money wasn't right.
Also, with the availability of prescribed suboxone, the whole clinic-thing became as much a social meeting as anything else.
Cops are looking for easy busts to pad their arrest numbers for promotions and raises, and trolling the clinic beat is an easy, safe way for them to achieve this, as pathetic as it is.