User blog comment:Fimber/Things that went wrong in "The Death Song of Uther Pendragon"/@comment-5102537-20130604105953/@comment-5674726-20130605173825

''I suppose you're right, ReganX. However, they could have clarified it all if they had just mentioned at some point that Nimueh and the Questing Beast were responsible for the particular law of magic that a life had to be taken in exchange for another. I'm still not sure whether this was their intention or whether the writers/showrunners simply ignored it due to plot convenience.''

A reasonably straight-forward explanation at their disposal was that different types of magic work differently. Using human magic, the Cup of Life, etc. to heal somebody of a mortal wound or to bring a life into the world that isn't meant to be there means that the Balance of Life and Death will seek to right itself. Dragon magic and Sidhe magic operate according to different rules, and don't carry the same price for healing. I can't imagine that they wanted to suggest that Nimueh was so powerful that she was able to write a new law of magic.