They wouldn't be moving, they would be expanding into the US market. Its a win-win. Hyundai get's access to a previously closed off market due to cabotage regulation. And we get modern ship building. The only loser here are established ship builders who would be forced to modernize and compete. But long term that's a good thing for them too.
As an analog, I believe South Korean terrestrial armaments companies are doing the same thing with Poland, in order to gain access to European buyers. (Granted, Poland also has a labor cost advantage in Europe)
