is substance abuse class mandatory? even if that's not your charge

We would consider taking a substance abuse course in prison a "good thing". Judges can order the course be taken or the offender can be fortunate enough to be invited. These courses are more about behavior modification than treating an addiction. Everyone that is incarcerated SHOULD try to get into these course. In prison, getting illegal drugs is highly unlikely so most offenders are not in the middle of an addiction which makes taking the course even easier. Some facilities will off time off your sentence for a successful completion.