The pickle that the cruise lines are in is this - they designed their loyalty programs to never expire - so as time goes one eventually every cruise aficionado will be a diamond member.
The airlines have programs where you have to spend your points for rewards and eventually to start over. The cruise lines don't do this.
And so - as they have discovered - people who join these loyalty programs tend to be the most detailed-oriented people in the first place, so they make sure they get the best of every little thing the loyalty club offers, and it adds up for the cruise line.
Or to put it another way - some of those people (not all) are like the people on the other site who tend to believe that since they are loyal customers they essentially have a seat on the board of directors and can boss the company around.
Every year they have more & more people like this, drinking up complimentary cocktails, using the laundry services, getting upgrades, etc, etc. They will have to reduce benefits some day, or else find some way to make you "spend" your rewards points.