Because there truly is no theme here. It's people dressing up as what they percieve to be "African". I wonder How a real African would have felt at that party. Would he/she have felt closer to home. Maybe admired the respect and attention the partiers paid to his or her culture.
It' supposedly an "African Costume" party. There are plenty of non blacks in africa. and plenty of costumes that don't involve skin.
You are looking to hard to see racism where there is none. If they came as an white African, would there be racism that they DON'T dress like the "norm" and be black?
Would your reaction be the same if, let us say, I threw a "Asian party" and my girl friend came dressed as a Geisha? Or if I came and made my eyes look like a normal Japanese man (not sure what you call it in English)?