It's also worth noting that one of the themes of District 9 is the construction of criminality. I found it very hard to look at the aliens, who were clearly shown being pushed in their status as criminals, and not see the Nigerian criminals (who were never seen outside the same ghetto to which the aliens were restricted) as a commentary on the modern situation in South Africa. I can't tell you whether that was how it was intended, but that's how I saw it.
Hollywood sometimes depicts urban gangs as white or multi-cultural for fear of being called racist. The result is a lie about the geography of class and race in the US: Gangs reflect the regional nature of poverty. The Nigerian gang in District Nine is troubling to Americans who focus on race. But the writer, clumsily or not, was telling a truth: gangs exploit whatever they can, and if the new market is space aliens addicted to cat food, they'll supply that need. If District Nine was remade with an American setting, the gang should be black or Hispanic if the ship stops over a city, but if it stops in the country, the gang could be whites who found a more profitable product than meth or marijuana.