This is strange, since no other P43/G43 boards have this limitation (and the P43/G43 chipset does not have this limitation, either, according to Intel's own technical specs). I am wondering if ASUS actually used a P41 chipset in this board or just re-used a PCB designed for P41 without any modifications or changes.
Its not a limitation of the P43/G43 chipset, so it can only be a limit imposed by ASUS design.