Wow, we are talking about a period with established international law, and the way you defend your position is by going back to the 17th century?
In the US' case, there were a long series of treaties. From the treaty of 1632 and the treaty of 1640 signed between British settlers and the Native Americans all the way to the Fort Laramie Treaty of 1868. In many of the early treaties with the British, the tribes involved agreed to each became tributaries to the King of England. All those treaties of course doesn't justify Britain and the US' treatment of Native Americans, but for a while, Native Americans actually had a pretty decent relations with the British, especially because the British were curbing expansions into Native American territories. At least in both the British and the US' cases, they both recognized there's some rights that native inhabitants deserved and it was reflected by the fact there were treaties and the content of many of the treaties recognizing their traditional territories.
The British declared Terra nullius when they got to Australia, but it's a bit silly for the KMT to decalre Terra nullius on Taiwan in 1945, and that's exactly why they didn't and had to make up elaborate lies to justify declaring Taiwan a province, taxation and military drafts.
I guess everyone goes for complete political realism when it suits them. That's why liberty and freedom is fought, not given.
By the way, the Americans even signed a treaty with Taiwan's Paiwan Aboriginals, in the aftermath of the Rover incident. Le Gendre negotiated with Chief Tauketok to guarantee future shipwrecked sailors raising red flags wouldn't be killed.