I also agree with the question why they have to kill off each new black guy like that
My theory on why they keep killing people is because in the books, Rick's group at this point isn't too big or strong - they are almost always outnumbered, so on the TV series they don't let the group seem dominant either.
The producers may also want to have a representation of other races on the show and are trying to represent the population of the state of Georgia to a degree (about 30% of that state is black), as that's where the location is set right now, while at the same time introducing new characters that are not in the books.
In today's politically sensitive society, maybe they're damned if they don't, damned if they do when it comes to race. When a white person is killed off, it's not talked about in terms of race (just like it really shouldn't be). There's really not too many black people in the comics anyway, and T-Dog never was even part of it. I liked his character though and didn't want to see him or Oscar die, but it's a show about death. A lot of people are going to die. Very few are safe regardless of their race.