Britain engaged in a strategy of 'terror bombing', the deliberate murder of German civilians, during WW2, killing over a million people. If a Brit wears a lederhosen, is that offensive? What about a Russian? French folk?

The present is more important than the past. In the past, we've all killed each other in horrific, cruel ways. What matters is the present - Native tribes often still use the national dress they're depicted with (without much concern for accuracy in most cases) in important cultural folkways, and Native Americans in general still suffer from severe discrimination. For that reason, not past genocides (though the past genocides are obviously important to recognize and acknowledge), is tribal 'costume' inappropriate and insensitive.