I have little doubt that there is more to the story than 75 cents.
I have no doubt that the media is manipulated and almost never in favor of the cruise line. Tender companies are owned by rich people and any such increases rarely end up in the hands of the workers. They know how to play the same games Carnival does.
As for port charges, there were too many law suits by ambulance chasers and some are buried in the cruise fare. There is nobody at the port taking a head count - it is based on an estimation of average number of cruisers. The average number of cruisers is, of course, proprietary information.
I have also seen port fees for a given cruise decrease. I really doubt the tendering issue is related to TA commissions.