I've had quite a bit of exposure to foster kids and their situation - not as a foster parent, but nonetheless many kids/situations over a period of years. Personally, I hate the idea of separating kids from even "borderline-fit" mothers, but the reality is that people from The System can try and try and try to get some parents to straighten up, and if they're determined to be, or trapped into being, involved with drug use they either can't or won't change.
The rotten reality is that there is no good option for children whose parents can't or won't do what they're supposed to do.
I know of two cases in which little girls were in foster care, with their mothers "being worked with" for quite a stretch of time. In one case the mother did something that made the social workers realize that the mother was "a hopeless case". In the other case, the little girl was being prepared for, going through the whole process of, going home to be back with her mother. She was super-excited, and went off with the social worker with all her belongings. They got to the mother's door, and the mother wouldn't open the door. The social worker pretty much knew the mother wasn't in any shape to open that door. That little girl went around crying for I don't know how long - weeks maybe. After having been reasonably happy, very safe, and reasonably adjusted to being in a foster home; she was faced with mourning over having her own mother not answer the door after weeks or months of working toward that aim.
Improving things like screening, testing social workers and potential foster parents, and better training social workers to pick up on the subtle signs of a potential foster parent seeming "a little off" might help.
But, for kids whose parents won't/can't do what they should do there may sometimes be a loving, safe, solid, foster home. For so many others, though, there's just not a good option at all.
Having said that, there's no doubt that some kids would be better off being left with their drug-using/drug-buying parents. One problem is, though, that a lot of iffy parents get a lot of "passes" before the state steps in and takes children away.
I don't think educating these parents more is the answer. They know what's right and wrong and do what's wrong anyway. Substance users know what they're doing, but they can't or won't stop using anyway.