It's not about what they claim to be, it's about the legal provisions that may or may not protect them. I'm not sure of all the details, which wouldn't fit in an HN post anyhow, but the short version is that if they are picking and choosing what can appear on their platform, they're also liable for it. Facebook, among other companies, want to both be able to pick and choose what appears on their platform and to enjoy the legal protections that come from not picking and choosing, but the days of that arrangement working for them are clearly numbered. And in the case of Facebook, they're clearly going to go for choosing what can appear on their platform, so it's only a matter of time before they get sued about it.