It is only recently that the rights of all people were recognized and equality became the social norm. Before this, people tended to grab what they could from other cultures and this often included cultural treasures. Since we now recognize that it is ethically wrong to rob a culture of its treasures, we should return cultural treasures whenever possible to the original culture, and if they are to be displayed abroad do so only with the cooperation and permission of the original culture.
Whenever another culture gets hold of something from a different culture, there is a chance that feelings will be hurt and norms violated. The new culture does not always understand what the first one intended. That is why we should definitely have consultants work with us on any display of foreign artifacts.
The UNESCO 1970 Convention protects cultural treasure from any illicit import, export or transfer. But it also says that a state should take appropriate steps to recover and return the cultural treasure. This indirectly means that a country should prove their side in order to get their freedom as nothing is impossible. If a country is not determined enough to prove their cultural treasure then that means that the country does not care enough about their cultural treasure.