There are indeed cruise lines that add an on board charge to cover gratuities or tips. For instance, Holland America adds a daily per person charge called the Hotel Service Charge (HSC). In the sense that this is automatically added to ever account I suppose it could be considered mandatory but, as pointed out, a passenger can have the charge removed so it's not absolutely mandatory. The downside on Holland America is that if a passenger removes or decreases the standard HSC then the wait staff is alerted and are told to turn in any cash tips received from said passengers. Needless to say there's a lot of debate and discussion about how well that works or doesn't work but those are at least HAL's rules. Now if a passenger leaves the HSC intact then the passenger
can, strictly at their own discretion, tip individual staff additional amounts which the staff member is allowed to keep.