[notahacker]

Societal structure is not a constant. In China, for example, the government willingly admitted that 2/3 of organ donations came from executed prisoners (2009), as proposed by the original argument. A purely rational Chinese person probably wouldn't consider criticising this policy to be a means of maximising his utility, and it's not even easy to make an argument that abolition of that specific practice in isolation would maximise society's utility.