Oh, I understand the rationale...I just happen to think that this should be disclosed before the gratuities are pre-paid.
IF I had had a bad cruise experience, I would interpret Carnival's inability to immediately address my grievances as somewhat short-sighted and a "salt in the wound" type phenomenon. As I'm sure you'll agree, no one wants to have to go through more red tape (i.e. Carnival/Miami) to have a situation reconciled.
In any event, there wasn't a problem, so the scenario is moot in this particular instance....BUT, I think everyone should know before pre-paying that they may be setting themselves up for a more complicated situation relative to recovering their gratuities.