While Grand Turk IS a Carnival-built port, it's clear they are allowing other lines to dock there -- and presumably charging a pretty penny for the privilege. And why not? GT is one of the best purpose-built cruise ports around. Might even be some extra revenue for Carnival when they need it.
Carnival did this -- and might still be doing this -- with Amber Cove also, another nice port they built in the D.R. Other cruise lines were being allowed to dock there, likely because the new cruise port over by Puerta Plata had not yet been completed.
I've been on a Royal ship to Grand Turk, a big music charter (60 bands, 4 stages) on the Indy. At the time the Jack's Shack said it was the biggest ship ever to make port there. It almost looked like the stern of the ship was in front of Jack's. It wasn't....quite. 🙂