How's that? I make a photo, and free it. Say it's on Public Domain. How is X museum going to claim that photo cannot be used because i didn't aks them a permission they didn't require at that time?
It's bad enough that they require a permission for the photos done inside (the copyright of the air between the camera and the object not being dirty? xDD) but retrocactiviy would be a total nonsense... At least from a point of view not earning money from it.
It's not a question of morality, it's a question of law. Non-retroacivity is not guaranteed in al juridiction under all circumstances, and I do not know whether it would be in this case.
-- Rama