Often, the lack of personal attribution in those artifacts would be used as justification. After all, the 1000-year old art work outside a village's gates didn't belong to anybody per se, did it? The British publicised a significant portion of Indian antiques but often squirrelled away the best of the lot to England. With the decline of the British Empire, these made their way to American collectors and museums. There was no concern then that the historical context of those antiques would be lost.
Now, the shoe seems to be on the other foot. When Japanese museums decide to pay exorbitant sums for dinosaur bones (found only in North America) or for American Indian artifacts found in Florida, the money encourages the pilfering of excavation sites. This, folks realize belatedly, is a bad thing. There is a massive outpouring of concern that science and history are suffering. The barely disguised notion is that the historical context not lost when Shogun swords adorn Western museums somehow vanishes when bones found in North America find their way to Japan.
Please.
