It's a good question, and I think it parallels questions about efforts to attract more women into engineering. One answer might be that when people feel excluded, it can help to provide a subspace where they feel privileged. The hope is that by attracting an initial group, the field itself can be improved so that future people like them do not feel excluded. Carving out a niche for Afropunk may provide a path of entry for musicians create music that would otherwise not be created.

Another possible answer is that the goal is not to create "punk for black people", but to create (or describe) an entirely new genre. Just as one can have East Coast and West Coast punk scenes, where the participants aren't necessarily limited to their geographic origin, perhaps the goal is to have room to allow for specialization. I don't know if it is there yet, but where it really becomes interesting is when the Afropunk scene is defined by style, rather than ethnicity.

This article captures some positive aspects of each of these answers: http://www.ebony.com/entertainment-culture/afropunk-before-a.... It also includes a link to "AFROPUNK: The Movie", which has it's own answers.