Well, ultimately, yes it is so you spend money on the ship.
But they will just cruise at a slower speed between the ports. Its not that the distance is longer. The cruise line pays for the privledge of docking in the port. The longer they dock, the more $$ it costs them. The port time is dictated by that, so they cruise slower or in circles wasting time so that they arrive in port at the specified time. They also leave port at the stated time, AND WILL LEAVE YOU BEHIND if you are not back on board, because if they are not gone by a given time, they get charged additional port fees.
Its always about the money.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013