To be fair, none of the characters talked about this relative lack of magical energy outside of Callahan. Luna never wondered why her powers were diminished during the vacation arc, Nimmel never contemplated why it took more effort to do things while he was in the Archipelago. Heck, it was quite the opposite for him! I can only assume the King is blowing this loss of magic WAY out of proportion, and using it as a flimsy excuse to commit some unspecified crime against nature by harnessing the magic comet.