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.
Odd as it may sound, it would have to be “structured randomness” so to speak. Picking a slip out of a bowl would probably work—getting a reward only when the parent is in the mood to give one would likely not. The latter is just as random from the child’s perspective, but inconsistent parenting (or animal training, or employee rewarding schemes) is known to be bad at shaping behaviour in the desired fashion.