It is I believe as while PRs have many of the same rights as fully fledged citizens, they remain foreign nationals.
When you become a citizen you have to swear allegiance to Queen and country and become entitled to a Kiwi passport.
In a way you can live here as a PR forever and becoming a citizen makes little difference. I have a few friends whose country of birth doesn't allow dual citizenship. These people came here at a young age and are basically Kiwis in every regard but they hold a foreign passport.
1. You can vote as a permanent resident, but you cannot stand for the parliament. The previous Greens leader Russell Norman, born in Brisbane, who came here to write his PhD in 1997, had to apply for NZ citizenship when he was was elected as a list member for the NZ Green Party.
2. Also, more importantly, you cannot be deported if you are a NZ citizen, except for immigration offences (i.e. your citizenship was obtained fraudulently, etc).