Why cant aboriginals marry someone on their family or tribe?

Society's all over the world have rules regarding the intermarriage of close relatives (1) some reason that this can bring about birth defects (2) while in other society's it is done to spread the family's social and political influence. There are also an encreasing number of country's that have no problem with this at all. My gues is that in the case of Australian Aboriginals @ would be the biggest reason.