Should Prisoners be set Free When Their "Crime" Becomes Legal?

[Jenny A]

As a law student I'm gonna disagree with Jenny (knowing she's an expert in the field ) -

I was giving a philosophic opinion, not a legal one.  

When we make something legal (and assuming it was illegal before) it means that we decided something that was once immoral, wrong or harmful is now acceptable or at least ignorable/tolerable -

Not necessarily.  Many countries once required drivers to use the left side of the road and now require them to use the right side.  Tax laws change continuously, but not necessarily for moral reasons.

And changing the law may not be because we find an activity now acceptable, but rather an acknowledgment that prohibiting the activity actually causes the practice of the activity to rise, or creates so many other negative consequences that we'd rather put up with the activity than suffer the side affects of making illegal.

For this reason, keeping people in jail for something that is legal is completely contradictory with the principle of justice and in particular what in Europe we call the legalistic principle - No one shall be imprisoned by a crime that is not expressly predicted on the law and by the law. I don't think it's a reason for people to not obey the law because there are some things we know will never be legalised. This is, of course, assuming our laws progress (not regress), meaning that each new law is better than the older one.

There is no such principle under U.S. law.  Although, there is a principle that laws held unconstitutional may be repealed retroactively.  In which case violators would indeed be freed.  My personal opinion, not an opinion of was the law is, is that if we acknowledge that the law was immoral, a human rights violation, or unconstitutional, then people previously convicted should be freed.  In some cases they ought even to be compensated.  

But many laws are merely judgment calls in gray moral areas.  I see no problem in retroactively forgiving people by law.  But merely repealing a law should not have that affect.

For drug trafficking, I would not forgive because it would still be completely illegal under new laws - You could buy marijuana (assuming we legalise it) in shops, but individual people without authorizations cannot sell it on the street - Not to mention that trafficking usually involves other small/medium scale crimes

I'm not sure the results of that line of reasoning would always be for the best.  After all, you can't comply with regulations until they are passed.