I like the idea of pre-paying. I just bury it in the cost of the whole trip and forget about it. As a result, I'm much more likely to give even more once onboard.
But I don't like the cruise line telling me I WILL pay. That sounds more like a demand notice or a tax than a gratuity.
I'm sure it makes it much easier to track the true incomes of these poor souls who break their backs for our enjoyment.