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Their terms are 'well known' & are non refundable. Simple answer is, if you don't agree with their terms, don't book with them. The fact you agreed to this contract makes it legal unfortunately.
Perhaps Costa Cruises should be commended for their honesty. Too many cruise lines disingenuously refer to 'service charges' as 'tips' or 'gratuities'. But I certainly agree with your sentiment - the idea that service should be rewarded with a fixed-rate charge is clearly a nonsense! I agree with Wilba (as I so often do); choose a cruise line which offers a contract you are happy with and, better still, let the one that doesn't get your custom know why you have chosen to go elsewhere.
We have cruised twice with Costa and yes the 'tips' are compulsory and cannot be altered or withdrawn so we just factored into the cost of the cruise.....