Royal Caribbean and Celebrity have (at least officially) not allowed discounting of cruise fares for the past several years. The goal was to have the same fare offered across the board - by the cruise line, by an agent, or by internet cruise sellers.
Carnival recently announced they were going to the same policy, effective August 1. And now we can add NCL to the list, also effective August 1.
There are of course many caveats for group bookings, special sales, or deals pre-approved by the cruise line for an agent or agency which wants to offer a special promotion. Gifts can be offered but are limited to 10% of the cruise price.
For the most part, the new policy will align NCL with the others which means no rebating of commissions or selling cruises for less than the established price.