While I agree that $0.75 per person is cheap, has anyone looked at this form Carnival's perspective of almost a quarter of a million dollars per ship, per year (As pointed out by gree_rd's math above).
Imagine if Carnival said, "Sure, no problem, we'll pass that $0.75 on to each passenger and they won't even know." This is one charge from one port. Imagine all other ports deciding to charge an increase because they heard Grand Cayman got one, and Carnival says, "Sure, no problem."
Carnival is a business like no other and when possible, they will attempt to negotiate new charges where they can to protect their bottom line, their shareholders, and YES, even their passengers. If charges were to go to the other extreme (especially in this weak economy), fewer and fewer people would cruise, net reducing their bottom line.
Carnival is a business like all others.
Western Caribbean - Elation - April, 2015
Southern Caribbean - Triumph - October, 2015
Exotic Western Caribbean - Dream (B2B) - November, 2013 (Our Honeymoon)
Eastern Caribbean - Dream (B2B) - November, 2013 (Our Wedding)
Mexician Riviera - Spirit - November, 2011
Western Caribbean - Freedom - January, 2011
Southern Caribbean - Miracle - November, 2010
Western Caribbean - Liberty - November, 2008
Eastern Caribbean - Valor - October, 2006