The purpose of the system is to physically restrain those who would re-offend, until such time as they change their minds; To deter others from offending; And to deter victims of crime from embarking on vendettas.
Any imprisonment term beyond what is needed to meet those objectives is a waste of resources.
That makes no sense. It's saying that if you murder your parents to get the inheritance money, you shouldn't be jailed for it because you're not going to re-offend because there's nobody else you're going to be inheriting money from.
Rehabilitation is one goal of the prison system, but the punishment aspect is arguably a more important one. It exists to have criminals face the consequences of their actions.