It's the most convenient way of labeling the group of people that were inhabiting the continent before ole whitey arrived. Unless you want everyone to start calling them "The Asians that were living here before we started indiscriminately taking their land and killing them."
Apparently the people who moved from Asia to America were the first people to populate America either way, and once they'd started having children and raising them there they would have been natives from that generation on. You could say that's an argument against racism, but there are still different races and ethnicities and there's still discrimination against them.
Mexicans would be part Asian by blood, as most are a mix of the natives of the region and the Spanish settlers.