I definitely take your point. The correctional environment is certainly difficult for delivering mental health and substance abuse services because of the inherent tensions within the environment.
Having said that, others have done it, and there are innovative models. Within the last few years, the U.K. has developed a different model for providing services to prison populations. They've taken the responsibility away from prison authorities, and the National Health Service now provides prison-based mental health services.
There are innovative models for doing it, and it's being done in other jurisdictions.