This is from there web-site and is in your paperwork, just what part is confusing????
"Exceptional service is part of the Holland America cruise experience. Because you are our guest, it is our goal to make sure you receive the finest service possible. Our "tipping not required" policy ensures that the professional and gracious service you receive on board is truly sincere; that we are dedicated to providing the most relaxed cruise vacation, not simply providing service in return for tips. You are free, however, to extend monetary recognition if you wish-it's entirely up to you. (Company policy prohibits us from suggesting how much.)
Please remember that this "tipping not required" policy does not extend beyond the ship. In terminals, airports, ports of call and on special shore excursions, you should tip consistent with customary local practices."
It is just like in the real world, you tip for good service.
Barry