Some maps have diff-specific storyboards, so you're meant to experience those at least once. I don't see why we can't have full BG dim and skin disable on the other diffs, though.
I don't even think it's even because of difficulty specific storyboards, it's just so that you don't get an instant advantage on a map you can't pass, because disabling the storyboard/skin/whatever allowed you to pass it.