When the Germanic groups like the Angles and Saxons arrived in England, the Celtic tribes were forced off their land, and those newer groups were eventally conquered by the Normans. Those first Europeans who came here were following the patterns they had learned in their homelands. That doesn't excuse their treatment of the natives, but since the Europeans were immigrants themselves, it would be difficult to deny future newcomers. Setting limits on immigration began after that Manifest Destiny had been achieved, and the realization that there was no more new land to settle.
I just finished reading Dan Brown's Inferno, where the villain is apparently out to reduce the world's population by releasing a deadly virus that will kill off the excess. It seems climate change is not our only problem for surviving in the future.