It's usage innit? If they call her Princess Catherine then that's what she'll be called. Protocol went out the window with Diana. The Royal Household was forced to change a lot of things because of her. Also when Princess Elizabeth married Philip, he was forced to take the Windsor name, instead of Battenburg (or Mountbatten as they styled themselves). He declared he might as well have been a eunuch at the time. His uncle Louis was furious, since he had done all the wheeling and dealing to secure the marriage.