Because Facebook already controls what people say and see on their platform. But they choose to use that power to censor boobs and show you whatever will provide the strongest response.
Do they, or are they forced to do so?
And as I said: I don't like Facebook. I think they're shifty and probably do have an agenda. However, it's untenable to attempt to force them to censor anything that a government doesn't approve of, or hell, society doesn't approve of. Facebook has 2.2 billion active users which make post 510,000 comments, update 293,000 statuses, and upload 136,000 photos per minute. It is impossible for a company to go through all of them in order to censor those which break whatever law of the country it was posted in.
In certain countries, to people below a certain age? Certainly.