I think if they released too many seats to partners, it would make their Airpoints programme (with redemptions largely pegged to cash fares) look even less appealing. Many years ago when they changed the programme currency from km to dollars, they put themselves in a vicious circle in that they can't get rid of empty seats and also their programme creates no loyalty. Now they are stuck with it unless they make a bold move to change it back. I know they had been thinking about it but decided against it.