Its a thoughtful note but is not a cruise just like going to stay at an hotel or resort. If one said it was all inclusive then perhaps no tips are needed except for special requests.
I cannot control the industry but I do recognize gratuity is a reality and will follow guidelines unless service was really bad.
Interesting enough folks who have worked in hotels / cruise ships will tip 10-20 % more then recommended is my experience especially listening to my wife and daughter who worked in these areas.
What are you going to do when you order a drink and 15 % is automatically added per the brochure, remove it.
To me its a cost to my trip, to allow for.