See Mike there is no legit argument.
We don't tip/subsidize laundry staff in hotels. We don't tip/subsidize cleaning staff in restaurants. We don't tip/ subsidize the handyman in bars. Desk clerks at check in (both airport and hotel) flight attendants, the baggage handlers at the airport, Etc, etc, etc.
Why should it become our problem on a ship? Just because? That's not good enough for me.
If you argue there are people who serve you that you don't come in contact with, you will literally be tipping everyone.
The behind the scenes people argument just doesn't fly with some of us.