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.