My understanding of the weight rule of FAI is to indirectly restrict the use of fancy power system, as by the FAI rules, there is no limitation for F3A engine type ore size. The AMA weight rule likely is derived from the FAI counterpart and allows the US pattern community and industry to be in sync with the rest of the world.

By the way, the 115g extra allowance over 5000g from the AMA rule does permit one to use slightly heavy parts/components at non-master level competitions. So AMA does relax the weight limit a little bit.