Black guy here, I wouldn't be too surprised if those two had made the post. In my experience, black people love to make jokes out of Ghetto stereotypes for their own entertainment (particularly of the "hey lets make a ghetto version of this otherwise mainstream thing that everyone loves" variety). Beauty and the Beat (http://www.youtube.com/watch?v=B0Rau01wbeo) is a perfect example. It seems that there is a subtle distinction within this topic that a lot of people don't seem to get: Most black people don't identify with the Ghetto stereotypes. "Ghetto" Blacks are a small subset of African Americans that most black people like to make fun of. This is similar to the way a lot of white people in America love to make fun of the pig calling, banjo waving, sister loving country hick stereotypes. They are not offended by it because they consider themselves to be apart from that community.

We only become sensitive to the stereotype when we start feeling that the joke implies that we inevitably fit in to that category because we are black. This is why your alternate explanation (nerdy couple takes pictures, racist adds ghetto stereotypes) could be very reasonably considered insulting. In general, we are not convinced that most non-black Americans understand the distinction between the overall black community and the "ghetto" community, and this is why we are generally more suspicious of these jokes when made by a non-black person than by a black person.

TL;DR : Most black people don't identify with the ghetto sub-culture so we also think its funny. We only get offended when the joke suggests that blackness inevitably implies "ghettoness".