It's possible that Apple just decided to enable Internet Tethering support by default for non-official carriers that Apple doesn't distribute carrier bundles for.
Which I'd be very happy with
Only one NZ carrier has an official bundle, but all of the carriers with compatible networks are happy for users to tether. One of the reasons I've been holding off getting an iPhone is lack of tethering, so if this "fix" is true then I'm a step closer!