I would never justify someone passing, but why would you say it is understandable in "her day", the late '60s- early '70s when she was popular? That was at the center of the Black Power/civil rights movement and people, more than ever, were proud of their black heritage. And there were other Black models who were popular at the time. I could see if it were a few decades earlier but what would be her reasoning?
This was very artsy for Playboy. I believe this was her concept. Almost like a fashion shoot by Paris Vogue and W during their golden days.