We covered this back in August in this thread
. The culture in the script is stereotyped, but they're Orientalist stereotypes rather than African ones; the episode explicitly says that the Ligonian culture resembles Ming China. The screenwriters based the Ligonians on samurai culture and intended them to be reptilian, or at least played by actors of multiple ethnicities. But the decision was made to cast them as all-black and use stock African accents.
Most of the sources I've read seem to place the blame on the director, who decided to cast all the planet's inhabitants as blacks. Apparently his attitude toward those actors was unacceptable, as Roddenberry fired him midway into production and another director finished the shoot.
I think "most of the sources" are probably just repeating what Wil Wheaton reported as hearsay in his blog analysis
of the episode, as discussed in the earlier thread. The story that the director was so racist toward black actors that it got him fired is questionable when you consider that, starting the following year, he went on to direct 18 episodes of In the Heat of the Night
, a show with several regular African-American cast members -- apparently including Trek novelist and TrekBBS member Geoff Thorne
! Perhaps if Geoff sees this thread, he might offer some insight.