As you, and others have suggested, it's to discourage passengers from booking NCL-AMS in business.
The last thing the airline wants is to miss out on a £4k sale from someone flying NCL-AMS-JFK because they sold the last NCL-AMS business class seat for £200. Anyone looking to spend that kind of money might not be too happy about being dumped into economy on the NCL-AMS leg.
Far better to keep the business class relatively empty and try and win over some higher value connecting business passengers.