Recall the Communist witch-hunt in the cold war. That was done without retroactive policing and parallel construction. Just because they cannot imprison you for actions taken before the action was illegal doesn't mean they can't fuck you over in other ways. It's the thought police all over again.

Perhaps they won't get you for stuff you did today that is legal today.

But if you are still a carrot fetishist in the future, they can use the knowledge that you used to be a carrot fetishist to set up a sting operation. A little parallel reconstruction, they seize your phone and laptop, and next thing you know you're in prison in the future, thanks to the data they hoovered up today and will be in their database forever.

That doesn't protect you from targeted enforcement of other laws.

The US constitution prohibits unreasonable search and seizure on its face.