So is Superman, but I don't see no non white actors being cast to play him.
If I'm not mistaken, Tom Welling is part Native American.
And how many of those various cultures were European and black African?
Culture =/= Race. A person's skin tone does not have an impact on a culture's development and the culture does not influence one's skin tone. Rather, it's the amount of exposure to the sun that determines how dark one's appearance is.
In that sense, it's not that unbelievable that the very central-to-the-equator-ly positioned Fire Nation are dark where as the northerly, moon-centric Water Tribes are fair.