I won't disagree with you. We're very easily led and manipulated to conform to group standards. However, it is in our nature to protect ourselves from potential threats. How a threat is seen can be linked with how a culture is set up to deal with outsiders, but a tribe that ignores the necessity of territory protection wouldn't likely have lasted very long as a tribe in the first place due to other tribes with their own ideals for what they want for their people. Think about starvation, and what may have been done in times of strife. If an outside tribe has food, but refuses to trade for it out of lack of enough of their own resources, there could potentially be a violent situation that arises. We're above animals within the sphere of complex thought, but that just gives more context to what drives us.

I'm not going to pretend that I'm an expert on these matters, and until we've got a time machine and a reliable way to observe those various tribes of the Americas we can only rely on the information that is already available.