Why is a lack of response to punishment in adults considered significant in diagnoses? It seems that a lack of response to punishment is often considered useful in diagnosing conditions or symptoms. A good example of this may be in diagnosing psychopathy, in which a lack of response to punishment seems to be a defining characteristics. For example:


  Their behavior does not reform in response to punishment; they will
  impulsively commit crimes despite knowing the consequences they will
  likely face. They are among the worst of repeat offenders.”


It seems odd to me that punishment is considered significant, since in many cases a person we consider capable of making their own decisions may not agree that they have done something wrong, in which case why would the punishment be expected to have any effect? 

My question is, how is observing a lack of response to punishment useful in diagnosing any kind of psychological condition? Is the assumption that most people will respond in an expected way to punishment? If so, what is that based on?
In response to your first inquiry, lack of response to punishment, in some cases, may suggest the lack of conscience/fear. Many are diagnosed or have the symptoms of a sociopath; being that they think themselves as untouchable, greater than anything/anyone outside of themselves; they don't feel fear the way "norms" do, nor do they feel remorse for what they inflict on others. Therefor, they will find "reason" for any and all wrongdoing they inflict.
  So far as your second question, it is not so much that it is "assumed" that most people will respond in an "expected" way to punishment. Rather, the  individual that has been tested in the same arena as a diagnosed sociopath vs. the majority of people who feel remorse, exude morals and personal responsibility. 

http://www.cassiopaea.org/cass/sanity_1.PdF

"The Sociopath Next Door"- Martha Stout

http://www.npr.org/2013/06/19/193099258/inside-the-mind-of-a-sociopath
 I hope this is some semblance of an answer for you.

Update:
 In your words, sociopathy and psychopathy are speculative.
  All science, on the other hand is proven by trial and error. Subjects being tested and who bear on the "normal" end of the spectrum, is named so by the trial and error which is performed and proven from past subjects.
   Moving on, I will say that, speculatively, the majority of people will not hold up a bank, steal money from petty cash at their place of employment, or commit murder, due to their morals, conscience, or fear of getting caught due to the punishment they may receive.
   However, if there is NO fear, NO morals, NO conscience, there may be room for diagnosis.
  A person may feel they have done nothing wrong and that their actions are justified. Everyone has reasons for doing what they do. But the majority of people tend to feel remorse when they cause others undue comfort, cheated anotherin an extreme circumstance, or was the cause of a terrible negative outcome. For a person to feel they have done nothing wrong and that their punishment is unjust, happens every day. But it's the one who does NOT feel, nor  take personal responsibility(the blame is never theirs) who may, over repeated instances, be diagnosed with a form of psychosis.