Actually this is the opposite of racism. You see, L4D2 is the first equal opportunity player modeler. The purpose of including African American infected isn't to say that black people are mindless zombies, its showing that all races are equally capable of being mindless zombies, gigantic armed freaks, puke-spitting fatsos, long tongued individuals with lung cancer, and hoodie wearing creepers able to leap tall buildings in a single bound.
The truth is that after the release of L4D(1), a group of ethnically diverse gamers petitioned VALVe, disturbed by the fact that most of the zombies seemed to be of Caucasian/European decent. "I can't believe how far white supremacy has reached," one African-American gamer explained, "Black people are equally capable wearing hoodies and smoking."
/sarcasm, if it wasn't obvious enough.