Air New Zealand blocking seats

Several months ago I purchased a window seat in premium economy on a flight from SFO to AKL. The first three rows in that section of the plane have a surcharge. I tried to change from a window seat to an aisle and both the web site and customer service have blocked seat selection for those special rows and I could not even change my seat selection to the aisle.

Does this happen often? And why, would an airline block rows of seats when they want to add a surcharge to them?

If anyone has any information sure would appreciate it.

Lot's of potential reasons. Access to bassinette spots, the AirBed program, saving for FF with status, etc. I suspect it is #3.

I suspected holding them for frequent fliers too

In a lot of cases, airlines tend to block the first couple of rows in (premium) economy (depending on the size of the plane and how many people are booked) for frequent flyers but also for families who are travelling together but are not booked together and did not reserve seats in advance.