Airline baggage - piece or weight system

Hi,
Does anyone know why US flights or itineraries involving US flights use the piece system eg 2 x 32kg per pax whereas flights for the rest of the world use the weight system eg 20kg per pax in economy?
The only reason that I can think of is that the US CAA insist on this requirement and that all carriers flying to US must comply - is this correct?
Given the fact that the piece system also applies for trips involving US flights, why don't airlines make this the global standard - I guess the cost of the extra fuel load may play a part here.
Cheers,
Sahajesh.
Does anyone know why US flights or itineraries involving US flights use the piece system eg 2 x 32kg per pax whereas flights for the rest of the world use the weight system eg 20kg per pax in economy?
The only reason that I can think of is that the US CAA insist on this requirement and that all carriers flying to US must comply - is this correct?
Given the fact that the piece system also applies for trips involving US flights, why don't airlines make this the global standard - I guess the cost of the extra fuel load may play a part here.
Cheers,
Sahajesh.