Air New Zealand Air Points - Domestic Service Fees

Just got off the phone from Air New Zealand to change a flight over (M booking class, domestic express) and first i rang the gold line which rabbled on about how the service centre was closed and i couldn't press 4 for reservations, the electronic voice literally said goodbye and hung up on me. So i ended up ringing up Koru Club Priority Reservations and putting my gold number in.

They were demanding $15 service fees to change the flight,despite being on a fully flexible ticket, which i had already paid $20 in travel agents fees for [No change in fare level was required]

Why is this the policy ?

The Koru Club reservations line goes through to the main callcentre (at the head of the call queue). The callcentre charges service fees to make or amend bookings (including Fully Flexi fares).