It's not just Carnival. Many of the cruiselines are starting to add the tips on automatically. I know Princess does and I believe HAL and Celebrity do too. You can have the automatic tips removed and tip the old-fashioned way if you prefer.
We had ours removed on our last cruise because we chose to eat at the buffet most of the time. According to Carnival's breakdown, if you leave the automatic tips in place, only .75 cents a day goes toward the buffet staff and the rest towards the dining room staff and room steward. We felt that since we were using the buffet most nights, it was only fair that a greater portion of the tips should go to them.
Carnival (as do most lines, I think) does require either a credit card or cash deposit to cover expenses on your Sail and Sign card. However, you can choose not to provide either if you don't have them activate the charging privileges on the card. I didn't activate my charging privileges on my last cruise. Since I don't drink alcohol or soft drinks, didn't intend to buy any onboard souvenirs, and intended to tip in cash, I didn't need to be able to charge to my onboard account and didn't want to have to mess with getting my deposit refunded on the last day.
Just remember that if you don't have your charging privileges activated (with credit card or cash deposit), you won't be able to purchase anything onboard since cash is not accept anywhere on the ship (accept the casino).