It is true in the sense that their base salary is far from a living wage. They earn a few dollars an hour at the most. Gratuities were created to disguise part of the cruise cost and lower the advertised prices, on the back of the employees dedicated to specific guests, knowing that some guests will not follow the guidelines.
Since they have no bargaining power, the employees have never been able to do anything about it and make the best of the system, knowing full well they can easily be replaced. All major cruise lines operate that way. Some smaller all-inclusive lines do not bother managing gratuities and simply charge guests enough to pay their employees.
Therefore, anyone that chooses to remove or lower gratuities, sometimes justified by the fact they decide not to use the dining room, is in fact cutting the salary of their dedicated team. Those employees can’t make up the lost by other means since they are assigned to that table.