In a case were gratuities/tips are mandatory they cease being that and become a service fee.
Tips are never mandatory. Having tips added automatically avoids many problems, such as looooong lines at the Cashier's Desk or Purser's Desk the last day of the cruise, with folks wanting to cash checks. Also, for folks not wanting to be bothered with dividing cash into envelopes or multiplying the per day tips and hoping to come up with the right answers, it's a convenience.
Holland America used to have a "no tipping required" policy, but everyone tipped anyway, the same amounts as on other cruise lines.
I'm sure Celebrity can adjust or eliminate the automatically added tips if a passenger wants to do that. I would hope that if someone receives such poor service that they don't want to tip, that they have brought it to the Purser's attention long before it's time to tip.