As a quarter Indian, I can tell you that there really is no Indian Culture. That vanished when the Indians were sent to a reservation. What they have now is a combination of reservation life and Boy Scout training from about 1920.
What is the real key to this law? The tribes are growing smaller as young people leave to find a better life and marry outside thribe. They want to bring in every Indian child they can simply to give the "chiefs" more political power. The fact that it harms the child and deprives the child of the sort of life their adopted family can give them is incidently. Political Power is far more important ... at least to the chiefs and their government friends.