Considering some of the very provocative pics I've seen on FB, this does puzzle me. I"m sure its a reaction from their advertisers. But what a poorly thought through reaction it is.
I'll bet that it's not advertisers. I'll bet that it's from a branding/"facebook experience" person inside Facebook. Said person is probably confidently asserting that advertisers care, but ...
However, the policy isn't completely irrational and has some precedent. Does Playboy show breast-feeding?