The legal headache of punishing someone for doing something that was accepted at the time it was done. Laws and ethics change a lot over 50 years, would you want people in 50 years time judging you for doing something that you are doing and is accepted now?
That's fair enough but why, like I asked, does it only appear to be a one way street in terms of dishing out lesser punishments but not the more severe that have since been diluted?