It may be related to Carnival's vacation guarantee that allows you to bail at the first non-US port. Since St. Thomas is in the USVI, they would have to haul pax to ST then to The Bahamas before they could disembark.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015