I mean, by that logic, the US government allowing anti-vaccination sentiment to proliferate without legal/policy action (read: the First Amendment), is also "unambiguously harmful to society".

Except, the issue is way more nuanced than that. The reason this expression is enshrined as a protection in US jurisdiction is because we decide that free expression trumps every other social need — with the narrow exceptions as laid out in Brandenburg v Ohio (imminent lawless action).

To the extent that this is undesirable, if the political will to amend the Constitution is absent, then you'd also have a hard time convincing people that Facebook should not exercise the same principle.