Indeed, FB should probably tread lightly in the gray area. But don't let the existence of a gray area confound attempts to deal with the giant, growing, cancerous, black mass.
This content is part of the worst filter bubble of all time. You and I don't see it, because this content is not merely targeted towards the "highly suggestible" demographic, but carefully targeted away from the "might publicly object or be able to do something to stop it" demographic, which most HN readers are in. That anti-targeting allows showing much more scurrilous content than could ever be shown on TV.
Something like "ABCnews.com.co" is A) impersonating ABC and B) publishing objectively-false stories. If Facebook wants to put a warning next to it, or drop its share priority, or only accept shares with comments attached, I don't give a damn. That's not an entry into a partisan fight, it's the news equivalent of flagging a spam link.
I have lots of Facebook friends who sometimes share dubious or ill-sourced stories, and I wish they wouldn't. But I have a few Facebook friends in a very different group, who post whole-cloth inventions at the level of the dumbest conspiracies, or take Onion and Duffleblog articles as real news. It's a fundamentally different phenomenon.
I think until recently the algorithm making the decision has had a peculiar bias. It's had a bias for "engagement" and the most engaging content is often the most outrageous / incendiary / clickbaity.
So if Facebook doesn't "choose what's good for you", a default decision was already made: they'll show you what's engaging. But I think it's important to appreciate that default is still biased. It's just biased in a strange way that could have negative political consequences.
I don't have a horse in this race, since I quit Facebook, but I'd rather see Facebook do its unavoidable filtering in a way that is aware of relationships but blind to content; you see more posts from individuals and groups that you give evidence of caring about, without regard for what they say.