The prisoners in question are already having their life and time being determined by the law. If CONSENT is given then its the act of a human participant wanting to do some kind of good. In some instances it could be the only way to obtain conclusive data in medicine. In terms of death row inmates who are destined to die, their last opportunity for confirming elation and satisfaction in life to do something good could very well be doing this act of self sacrifice. The comparison is the individual contributing to the greater good in a collaborative effort or letting them do little more than idly using up their years.