I never said anything about 'their rules', I only wanted to know why things did not make sense to you.
Let me take a stab at why NCL, in their wisdom, changed their protocols. They aren't overly interested in whether passengers may or may not be displaying symptoms, they are more interested in making it easier for people to book and pay for cruises. By abandoning the fully vaccinated requirement this opens things up. Now, vaccinated people may or may not self-assess their fitness to embark the ship and may or may not be entirely honest with themselves and others about whether they should be on the cruise and that's a risk NCL and, apparently, all the passengers will have to assess themselves. The unvaccinated are still required to test beforehand and, I expect, that requirement will soon fall by the wayside if the new protocol works out OK... if it doesn't we'll just have to see what comes out of that.