Hey Imza, as I mentioned to you in an earlier discussion over at Ratskep, I think the problem here is just basic ignorance of what psychology is. People aren't interested in psychology because they think it's all wibbly discussions of the "unconscious" and wanting to have sex with your mother, and so they don't bother to read about the mathematical laws that underpin our thoughts and behavior.
Whilst I agree with your confusion over why someone would appeal to the DSM to question the scientific validity of psychology, I think you're being a bit unfair to the DSM. The recent versions are becomingly increasingly based on scientific evidence, and the criteria listed within it have been refined according to the best available evidence. The problem, however, is that people tend to view psychological disorders as being biologically-based, so when people say, "No cause have ever been found for a single mental disorder", what they mean is that no concrete spot in the brain has been identified as the cause of OCD or whatever. The obvious flaw in this is that causes of many psychological disorders have been discovered and found, for example, some forms of phobia, PTSD, depression etc. The confusing part for some people to grasp is that there can be multiple causes to the same problem, and there can be different behaviors which are a manifestation of the same problem.
But, as you say, discussing clinical psychology is irrelevant in a discussion on the state of psychology as a science.
His claim is also incorrect. Doctors routinely lock people up and put them in isolation when they contract contagious diseases to protect the life of the individual and society as a whole, and parents can be forced to hand over their children in order to receive life saving surgery or medical intervention when they refuse it provide it.
As you correctly point out though, even if it were true that psychology was the only health profession to do this, so what? It's a political/legal issue, and only related to clinical psychology, so it's confusing as to how it all relates to the science of psychology.