Re: In this case
Silly and pointless argument. The assertion of "ownership" in most of these cases is based upon political views and not a few items returned would almost immediately vanish into a black market. The "artifacts" - and BTW dinosaur fossils are not artifacts or artefacts - could be returned, yes. But to whom would you return them? The current government of the country that presently controls the geography from which they derived? ISIS would appreciate that. The Elgin Marbles - hmmm, Greece, where they came from, or Turkey, the polity which controlled the chunk of geography at the time? Do we make an exception since Turkey is now a democracy and not the Ottoman Empire? How about we ask Athens to secede from Greece so that we can return the Marbles to the city state from which they came? After all, when the Elgin Marbles were carved there was no political entity called "Greece." The "return" arguments are a perfect example of the real meaning of "politically correct."
The sole purpose served in these types of arguments is the process of "current politics," and the feeling of "moral righteousness" by the advocates. And, "punishing" many of those 18th and 19th c "collectors" ignores the reality that they often acted to preserve items that would otherwise be irretrievably lost. The Egyptian government of the 19th century for instance was staunchly Muslim. It had no use for ancient ruins, or ancient statuary as anything but building materials, any more than the governments of Egypt earlier had any use for the Library of Alexandria for anything but a source of fuel or the Taliban had use for the Buddhist images of Bamiyan.