Cruise ships I believe almost always have foreign registry.
The US government CDC can no more tell a Norwegian ship what it can do than it can tell the owner of a bar in Oslo to wipe down a bar stool.
Assuming they are in international waters. But are dock different rules apply, I believe, but will stand corrected.
That would be true only if the ships were not sailing out of US ports. The bar in Oslo does not serve drinks in Miami.
uh.....they leave from US PORTS! BUT the CDC SHOULD NEVER HAVE HAD THIS CONTROL!! NEVER!