I don't think it's a regulation per se, just that it's the USA industry standard.

It's always been my understanding that if a PNR included US flights that a higher allowance applies to the whole itinerary.

Didn't know about QF, but I know Aer Lingus has something similar, in that if fly them transatlantic, any EI flight two weeks prior/post will allow the higher limit on a seperate tickets. Just show them your transatlantic e-ticket.