I've discussed this in other posts, but here goes;
The cruise price that you see advertised is actually made up of two fares; the cruise fare and the non-commissionable fare (or NCF - these are port fees and other fees).
Since the ports charge a fee based on the number of people (not the size of the ship or twice for one person in a double cabin), the proper way to charge the port fees to a single traveling would be only once.
So, while almost all cruise lines will charge double the cruise fare (sometimes there are specials dropping this), the NCF's are charged once or twice depending on the cruise line. If they are charging it twice, then this is just a way for them to falsely make more money.
NCL is one of those cruise lines who charge the NCL twice.
Now, here's the kicker with NCL:
If you book one of the new single cabins on the Epic, they will only charge you the NCF's once. HOWEVER, if you book any other cabin on the Epic or any of their other ships as a single, you'll pay the NCF's twice!
Go figure their reasoning for that one!
I understand cruise lines charging the cruise fares twice - after all, they can make more money for two people in that cabin. But to falsely charge the NCF's twice when they need to charge it once is just plain wrong.
Hope this explains it.