Paying a drug addict to get clean isn't rewarding good behavior so much as rewarding the cessation of bad behavior. This has clear problems. For one thing, it isn't random like the "follow the speed limit for a chance at a small reward" scheme.

A true equivalent would be rewarding random people for not being on drugs, including the population of former addicts that have since gone clean. Being on drugs would be a garantee of not getting this reward.