[PeterisP]

You can't be convicted for something that was legal when you did it but was made illegal afterwards, so retroactive prohibitions don't apply.

However, it's not symmetric - lifting prohibitions can be done retroactively, to not prosecute people for things they did back when it was still prohibited.

Unfair, arbitrary prosecution may violate someones rights, but unfair, arbitrary immunity can not - there's no right to get someone else punished.

[philipodonnell]

Even when seeking restitution?

[vageli]

This is the same play used in the early aughts with at&t. [0]

[0]: https://en.m.wikipedia.org/wiki/Room_641A