"Sorry, we weren't enlightened 20 years ago, so you're going to stay in jail" is an embarrassing cop-out. This is a systematic flaw in the legal system, and is unjustified. If it took decades to figure out that marijuana was wrongfully illegal, then those persecuted under such a law should be removed from incarceration at once. How can it be any other way? This is common sense.
Keeping people in prison for actions now known to be harmless is a terrible and embarrassing flaw in moral structure.