Retrospective punishment?

KiwiPundit has a good reply to those (like NZpols and (apparantly) Deborah Coddington) who think that retrospectively altering parole procedures for paedophiles isn't a violation of the Human Rights Act.

My opinion? I'm against retrospective law changes, especially of this sort. Our justice system is predicated on the idea that criminals serve their time, after which they've "paid their debt to society" and are released. Piling extra penalties on whenever someone unsavoury is about to be released from prison undermines this. It both breaches the prisoner's human rights (and contrary to ACT's opinion, they still have them), and it gives them no reason to reform. In other words, it's both immoral and stupid.

Under the current system, if we put someone in jail, they're going to be released sometime. Even paedophiles and murderers. While we can impose parole and post-release conditions, they also have to end sometime. Unfortunately, the "hang 'em high" brigade, with their obsession with revenge, are unwilling to accept this.