Apparently what they remove has to do with whether there's nipple/areola showing. FB bans female nipples, basically, so if the baby's head is covering the nipple it's fine, but if there's any showing and it gets flagged/reported, they take it down.
Lots of prudes here, and they can't seem to get what they want to be prudish about straight - drunk people in mini-skirts and bikinis in provocative poses make fine FB photos, apparently, but a mother feeding her baby does not - cos gasp NIPPLE might be on show!
It's loony, IMHO.