The theory from maritime law is that the asset would otherwise sail beyond the jurisdiction of the Court. Whether there should be some due process is another issue. Carnival is rich enough to pay the judgment and has sufficient US connections that they would not try to ignore the judgment.

What happens if the Defendant was a Venezualan company and if you let the asset leave port you'd have to try to get Venezula to enforce a US judgment? The US has moved back and forth on the level of due process required before assets are seized. The German passenger had to make some showing of merit to obtain the order. The US judge probably issued a "show cause" order which said that Carnival had to show up in ten days or so and demonstrate why the order should continue.

Close your eyes and imagine being the plaintiff with a shakey defendant with minimal ties to the US. You'd want the order. Now again close your eyes and think about the gun being pointed at your head when a highly valuable asset is being "held hostage." Like many things in life, there are two sides to the story.

As to the theory for US jurisdiction, we'd need to know alot more. Remember how the victims of Bopal got fleaced after the US sent the Union Carbide suit back to India.