The networks cannot really tell the difference but maybe some still have T&C's that warn against tethering? I guess some want you to buy data separately for a different device.
They don't need to. Most phone OSs these days ask the network if tethering is OK, and disable the function if not. NZ carriers do not make use of this, but I know Virgin Mobile in Aus disables tethering if you're on prepaid as an example.