Personally, I think it's up to the addon author on what they want to with their code. Sure, it'd be nice if everyone released the way you wanted to but that's not the way life works.
Otherwise, I'd have my way and knowledge would be shared between everyone and there would be no closed source.
Anyways, that's just my touch on it. People are picky about their code and they may not want someone to just "take up the torch" after they have dropped the project.
Though, unless you live in a country that has an issue with public domain, my code is always available for free and you can even steal it, claim you wrote it and then sell it. I don't care.
Just not my WoW addons, Blizzard wouldn't like that.