They were told to children to scare them - morality lessons and so forth.
Very true, but I don't see how this doesn't also apply to Star Wars. Lucas said they were not only entertainments, but lessons for kids, the kind of stark black and white type of morality plays that kids weren't getting then, with everything noirish and in shades of gray. I think kids SHOULD be scared that the kid they identified with (Anakin) ended up burning up next to a lava river--that's what happens to bad kids who ignore their teachers and get impatient. Those tales were told to scare children, yes, but they were also told to entertain as well. It wasn't exclusive--the learning sticks better if you entertain them. Grimm and Aesop knew that very well.
But yeah, I've always thought Rock-A-Bye-Baby was sort of macabre, but then again, that's the sort of neurotic thing only adults really worry about. Kids seem to handle those kinds of suggested horrors a lot better than adults do.
But R-Rated Star Wars is an oxymoron, to me.