Because it is the expression, not the idea.

Somehow, that concept gets lost when Disney is suing somebody. Still, if you can show you are doing it from Grimm's and not Disney (not hard, if you don't mind a hard R rating, X if you go back to the earlier sources) you can get away with it. Anne Rice (AKA Rampling) never got sued for her version of Sleeping Beauty. (Skip it - bad even by porn standards.)