I suppose it's a sort of shallow reference to keeping a child locked up in a tower away from the world. Not sure it's a great comparison since the woman raising Rapunzel disowns her when she discovers she's been sleeping with a prince, and blinds him. But they find each other by her voice, her tears heal his eyes, and he takes her back to his kingdom. So I guess the moral is if you keep your innocent child locked away from the world the only people who will make an effort to find them are gallant princes.