"the USA fingerprint all air travellers whether they have commited a crime or not"
"I don't know about international flights, but this isn't true for domestic flights. I fly on a reasonably regular basis and have never once been fingerprinted."
It is US immigration law, so it happens on arrival to the USA. However, the USA is unusual in that there is no concept of transit. Most countries you can fly into the country and remain air side -to catch a flight onto a third country without needing to cross the border. However, this is not possible in the USA. If you are on the Air NZ flight from london to Auckland, you need to get off the plane in LA, enter the USA (including finger printing, having a valid US visa (or visa waiver) and then leave the USA to get back onto the same plane to continue your flight.