There is less evidence of cannibalism among Native Americans than was documented by the Romans in connection with the indigenous European Druids, and Druid human sacrifice was more extreme than any North American tribe's. I think you are confusing torture of enemies with slavery. Slavery was not practiced by Native Americans anywhere on the scale as that of the US, and neither was the state of a slave as immutable as it was in the US. It was not organized, no one had 200 slaves, no one filled ships with hundreds of slaves over many centuries; it was not an economic industry. Movement from slave to tribe member was far more fluid and more common than anything seen in US slavery practices. There is no comparison between slavery as practiced by tribal people in the world, and the institutionalized, transcontinental, mass scale of European slavery.

Who NEEDED to pass those laws? Things were not changing. Slavery had ended a hundred years earlier, but Americans weren't changing.