Princess is blowing smoke about being the only cruise lines to have a license to buy directly from the artists. Any company can purchase from any artist any time it cares to go to the trouble. But many people fall for that.
As far as tips are concerned: I prefer to tip the employee in person, but I am entirely aware that they have to kick back to their supervisors who aren't in direct contact with passengers. Sometimes I give an envelope as well as a separate tip in hand (and in private) if I feel it is appropriate.
If you really want to be confused: read Holland America's 'no tipping policy'.