Penal Substitution

Why don't we allow penal substitution in society? Why is it, for example, that we don't allow a husband to serve a wife's gaol sentence? Indeed, why don't we allow someone to pay another's fine? If I just write a cheque to the magistrate's court and say 'that's to cover the next few people that you fine' they won't accept it. Of course, I can actually give the money to the guilty person and let him or her pay the fine, but this isn't analogous to the Protestant doctrine of penal substitution; in fact, it is analogous to the Roman-Catholic idea that Christ makes satisfaction for us and we offer that sacrifice to God in the mass.