Paulusthemighty, the gratuity charges also go toward paying the back-of-house staff that you don't interact with. I had a very informative discussion with a staff member about this on my NCL cruise a few weeks ago. So while it's great that you want to tip those who provide direct service to you (and you absolutely should!), there are a lot of people behind the scenes who help make the service you're receiving possible that will never be in a position to receive gratuity directly from you.
The notion that this is "robbing people off" is simply inaccurate, and, frankly, a little cruel. Given the sheer number of employees working on a cruise ship who are helping to make your trip possible, a daily "tip split" of all direct gratuities received would be a logistical nightmare. It's not a corner cafe with 12 active employees who can quickly settle up at the end of a shift.
As the previous poster said, think about the people who laundered and pressed the cloth napkins you're using in the dining room, the person who cleans your dirty dishes, the person who's carting provisions to the kitchen that's preparing your food, the person who's breaking down and incinerating your garbage, etc. There's a lot happening that the cruise companies work very hard to keep out of sight for vacationers. But that's no excuse for being oblivious to the fact that there are a lot of people you don't see who are working to make your experience a positive one.
If you haven't done so, I'd highly recommend taking a "back-of-the-house" tour on your next cruise. It's a perfect sea day activity. We just did one, and it was fascinating -- and enlightening -- to see the behind-the-scenes operation that makes it all possible.