Actually, it occurs to me that it shouldn't happen on soups, either. If a song's already in a soup, then it's because it already matched the soup's criteria, meaning that the field is already filled out properly. But if the field is not relevant to the soup, then you don't want to propagate it then, either.

For example, if you have an artist soup and you find the U2 one, you don't want to change all of the tracks to say "The Joshua Tree". And there's no point is propagating "U2", since they are already that way.

The only thing I can think of where this might be helpful is on tracks or playlists that don't already have the modifed field filled out. It could also help to rectify existing playlists with mangled tags, but then jEmplode should ask in some way.