After all, the mandating judge, teacher, doctor, or social service professionals all have the best interest of the client in mind. So do the clients. It seems so obvious that a father should stop drinking and stop verbally abusing his child who is not performing up to his ability. When asked in a proper manner, it is very likely that the father would say that he also wants what is best for his son. The only difference is that the father may not see the connection between his drinking and his son's difficulties in school.
Human motivation is much more complicated than what is the right or wrong thing to do.
The usual way of thinking assumes that the client's motivation is something that either the clients carry inside or do not. This kind of causal thinking tends to make practitioners to blame the father for the child's misbehavior. When the therapist has this mind set, naturally the conversation with the client will flow from this thinking and clients will resist having somebody's idea imposed on them. It's not difficult to imagine what follows from this kind of thinking. The problem lies in them or in their backgrounds.