The transfer of the power of "Ni!"

King Arthur is petrified of the Knights who say, "Ni!". He recoils when they use "Ni!" against him.

Yet later, when he tries to get information from the old woman, he suddenly has the power to use "Ni!" against her; as if he were one of the Knights.

Did the sudden blast of music from nowhere give King Arthur the power? Did he have some kind of divine right because, after all, he was a "King, you know"? Was the old woman too ignorant to realize the "Ni!" was coming from an incorrect source?

And why was Roger the Shrubber so up-in-arms over the whole thing? In an indirect way, the Knights who say, "Ni!" gave him some nice profit for his shrubbery business...