This system based on race is akin to classifying people into ethnic groups. I'm quite sure that in Australia, authorities can also ask you at some point if you're an immigrant, and where do you come from. To my knowledge, the law doesn't treat Australian Aborigines the same as immigrants or Australians of European descent. It's true they're all humans, but different ethnic groups (or races in America) may behave differently when it comes to crime, suicide, mental health, employment status, literacy rate, number of children per family, etc.. That's why we often hear in the news that African Americans have the highest crime rate, or that Native American men commit suicide more often than men of another ethnicity. The reasons for this are historical: black people in America are the descendants of slaves, Australian/Native American Aboriginal cultures have undergone ethnocide, having (lots of) children is encouraged in certain cultures/religions, etc..