I do not think the law should ban a symbol. (There may be some special contexts where a symbol may be construed as a threat, but that would be an exception.)
Lu asked for a definition of "free expression". From the thread, I assume it is the legal right of an individual to express herself in the way she chooses, in speech, writing, dress as long as she is not violating someone else's rights.
Suppose the law does not ban it, is it still in good taste?
I've never been to a costume party. I do not know the rules of ettiquette. Does one have to wear a custome of someone good? Can one not come dressed as Torquemada? I suppose it depends on what the host specifies.
I see no reason to assume that the prince acted in poor taste.