Ah, okay. I misunderstood. I thought you meant that your NCL flights were with one airline and the ones you booked were with another.
So if your return flight is really one-way then I guess that would not be throwaway ticketing, at least from the airline's perspective.
When you say NCL wants to charge you 100% do you mean on top of what you already paid, or just that they won't refund you anything for the flight that you don't want to take? No refund seems quite understandable.