"[T]he practice of penal substitution in other scenarios seems wrong. We do not think it good for the mother of a convicted rapist to serve his time in prison. I propose that the reason why such a transfer is morally counter-intuitive is that while the victim still has the right to transfer the punishment, the likely good ends of such punishment would not be served by such a transfer. Given that deterrence and prevention are the main potential goods of criminal punishment it is probably never good that such a penalty be transferred, for there is little hope of achieving these goods through a transfer." - Steven L. Porter "Swinburnian Atonement and the Doctrine of Penal Substitution," Oxford Readings in Philosophical Theology: Volume 1: Trinity, Incarnation, Atonement edited by Michael Rea (Oxford University Press, 2009): p. 326.
Just when we thought Porter's theory of punishment was retributivist through and through, it turns out that he is a utilitarian after all. What else could he mean by saying that: "deterrence and prevention are the main potential goods of criminal punishment."
Clearly, there is some confusion in Porter's mind here – and no doubt he would retract this statement, and revert to his usual retributivism, if its consequences were pointed out. But it is worth taking him seriously at this point, just to expose the trouble that a utilitarian version of penal substitution will encounter.
Let's suppose that the father of a convicted rapist agrees to serve the time in prison on behalf of his son. No one but the judge, the son and the victim knows about this transfer. So far as the public are concerned, the right man has been jailed.
Let's also assume that the son is duly chastened by his father's amazing sacrifice, and turns his life around. He does not re-offend, and goes on to live his life as an up-standing citizen.
In such a case, it looks like the penal substitution has brought about the "main potential goods of criminal punishment". We have both specific and general deterrence (so far as that is ever possible) in the bag.
An innocent man has been punished, with the full knowledge and authorization of the legal system. This is outrageous. It makes no difference at all that the transaction was voluntary, or that the good of deterrence was achieved.
It is morally wrong to punish the innocent. Period.
Private citizens, even victims, can offer themselves up as sacrificial lambs if they like. But our legal officials have the right to over-rule this offer – as a matter of justice, and as is their duty as upholders of the 'rule of law' values of impartiality, independence and consistency. As Andrew Ashworth puts it:
"Just because a person commits an offence against me, however, that does not privilege my voice above that of the court (acting 'in the general public interest') in the matter of the offender's punishment. A justification for this lies in social contract reasoning, along the lines that the state may be said to undertake the duty of administering justice and protecting citizens in return for citizens giving up their right to self-help (except in cases of urgency) in the cause of better social order. " A. Ashworth, "Responsibilities, Rights and Restorative Justice" Brit. J. Criminology (2002) 42: 578-595: p. 585.