I also agree that automatic/mandadory tipping is taking my freedom of choice away. I feel that Carnival will use this to cover additional expenses without incurring any out of pocket costs.
I also believe that in time the cruise director will not hesitate to inform passengers that they may feel free to give an "extra" tip to those personnel that have provided "exceptional service". The envelopes will continue to "show up" on the last night, and over time people will no doubt feel compelled to place money inside, regardless of service. So in time, we could end up tipping twice.
Each passenger should be allowed the choice on whom they want to reward and how much they wish to give.