The cruise industry is not a standard service industry and never has been. Too many anti-tippers get hung up on semantics and use it as an excuse to not participate, or to substitute their own variation. Whether tips/gratuities/largesse/whatever, the extra money has always been part of the hard working crew member's compensation packages. It is not based on the price of the cruise, meal, or anything else. It is an optional (unfortunately) cost per person.
Why tip someone you don't see? Because it is always a team effort. Without the team, you have no service.
NCL has even changed the name to service charge, and some people still don't get it.
On some short cruises (some of the 3 day?), I believe Carnival has even required gratuities to be prepaid and some still try to weasel out.