I get what OP is referring to though. On some of these nat geo specials they will be in areas where the people are not Black and it appears more care is taken to not show them completely nude.
I view it the same way as how Hollywood has no problem throwing naked breasts and even full frontal female nudity in any movie or any show on premium TV (Showtime, HBO, etc.) often when its extremely unnecessary to the plot or even the scene itself. But you will rarely, if ever, see a penis. In fact they make it insulting to an extent that they will show an entirely naked woman and juxtapose it with a man that has lamps, books, whatever conveniently in the shot so you can't see his nudity.
In the end, I think they know which gender they want to commodify, and they know which races whose 'decency' they don't care to preserve.