What is The What- Robbers

The novel/autobiography by Dave Eggers and Valentino Achak Deng, What is the What begins with the robbery of a Sudanese immigrant Achak by, ironically, a couple of African-Americans named Tonya and Powder. Although a robbery itself is not a laughing matter, something struck me as humorous with the characters themselves. Irony is just funny to me. I can't help but notice Tonya's proclamation of "helping a sister" is achieved through theft, let alone theft from an African African, their brother and supposed comrade. They attack Achak Deng with slurs such as "African" (and other derogatory swear words) when they themselves are some part of African. Tonya and Powder steals from someone who has less materially but more internally, which leads one to think who really has the power? Who really has the one up in the situation? Why did they need to tie Achak down when he proposes no power struggle?