I would have to disagree regarding calling the tip a service charge. I have worked as a waitress in the past and many restaurants do this pooling as well. I personally do not think they should pool the tips because some do deserve more than others. I will never forget having to split my tips with someone who dragged their feet all night.
My suggestion is if you do not like having your tips added to your s&s account all it takes is a call or visit to the Pursers desk to have it removed. We did this when we sailed on the Pride. I prefer to tip personally as as much as I feel I want to. Besides have you seen the amount ......$9.75 per day on Carnival....that includes the room steward, head waiter and asst. waiter........that is not a very generous tip for the service these people give us. A waitress/waiter makes more on a Denny's breakfast!
Now knowing that they pool tips I think I would leave the tips on the s&s card and slip my waiter, asst. waiter and room steward additional money in their pocket that they don't have to share.