You need to read the fine print on the NCL policy. It is NOT as wide open as it appears, its just marketing.
Their policy clearly states, anyone can book a fare, BUT it is the responsibility of the PASSENGER to determine if they are eligible to visit a destination, and that all requirements have been met. When you show up to the port for embarkation, if you are disqualified to visit a particular destination due to testing or vaccinations, you WILL be denied boarding at your own expense.
Royal is actually doing it the right way, and making sure folks have a little guidance, and don't get themselves into a bind.
Also, take a look at the liability that NCL is requiring of the unvaccinated. Long story short, you test positive mid cruise, you are on your own, and can be discharged from the ship at any time, in any place.