First, Facebook has been under massive scrutiny recently for not exerting more control over combating fake news, fake accounts, posts that depict or could incite violence, etc...

Second, because she's leader of the BLM movement, I think everyone would agree that she is at a greater than average risk for being targeted with violence. If your position at a company requires you to identify such conditions and attempt to reduce this risk if possible, you'd likely feel a sense of responsibility to block the article.

Finally, given their recent string of negative media attention, finding themselves in the spotlight with accusations that they contributed to her harm were she to be attacked, is something they're likely keen on avoiding.