I agree with you, but just because we believe that doesn't make it common sense or "just." To someone else, the idea that someone who broke the law would be released simply because the law changed is a violation of justice (the ex post facto argument). The fact that I personally think it's an unjust outcome to leave them in jail (or throwing them in jail in the first place), doesn't mean that my opinion is the only correct one (I'm not a strict moral relativist, but I concede they have some good points).

There are some values more fundamental than others. One might be that inconsistency or hypocrisy is bad.

One might stipulate that whether marijuana should be illegal isn't an absolute that can never change.

But if it is in accordance with our values that it should be legal now, and this is our best attempt at instituting just laws, then leaving people in jail for reasons we don't currently think are valid is relatively worse than any arbitrary rule about drugs.

Just because you don't know for sure what is truly and absolutely right doesn't mean you can't stop doing things you know are truly and absolutely wrong.

Agreed, I think this is where I generally fall as well. Pragmatism as a philosophical discipline is often criticized as lacking a purity, but at some point we have to stop philosophizing and actually make policy.

Side note: I love your username :-)

I think a lot of nasty laws we have are a consequence of voters not being able to distinguish between “I don’t like this” and “this should be illegal for everyone“