Individual miscarriages of justice cases are not always heard of by the general public, unless they are very high profile. Even with high profile cases the majority of the public only know of the case, by what they have read in the tabloids or broadsheets or the local gossips. It is only with the use of the internet that details of individual cases are being able to reach a far wider audience therefore more people are getting to hear the ins and the outs of cases.
I think that individual prisoners who are claiming to have been wrongly convicted and suffered a miscarriage of justice, who ask to take a polygraph are saying, help me, I have evidence that I did not commit this crime, for various reasons I can't use this evidence as it wont be allowed/doesnt fit the criteria or the evidence has been refused, I need to let the public know I have this evidence, people will probably not have even heard of me or my predicament but I am innocent and I will do anything to prove it so when given the opportunity to take a polygraph they accept it, not as a challenge, but by believing that if they tell the truth it will back up the evidence they already have that they did not commit the crime that they have been convicted of.
I genuinely believe that an innocent person will try anything to prove their innocence.