I was only reading about this yesterday. Part of me doesn't like the idea, but the reality is, Koko lives in captivity, and anything that stimulates her can only be a good thing. Presumably, even though she's technically a wild animal, there must be no perceived risk to the kittens.
I'm sure the keepers have nothing but the best intentions for the animals in their charge but, sometimes, nature will out as demonstrated in this article: