A carrier should not discriminate, especially politically, against those that use their service but then neither are they liable for whatever is transmitted using their network.
A publisher can discriminate, even politically, on what content they allow on their platform but likewise they are generally held to be liable for that content too.
Social media is currently enjoying the best of both worlds: they can curate like a publisher, but want to avoid liability for the content like they were a carrier.
I think in general the law is pushing them into the publisher category because there have been cases made against social media companies holding them accountable for the content posted to their platform. On the other hand the public in general tends to treat them like a carrier because they interact with social media as freely as they would a carrier. To underscore that if any censorship or curation occurs it happens AFTER the user has posted in most cases. This is not something that ever happens with publishers. If one writes an article for a publisher like the NYT then an editor will look it over and exercise editorial control over it BEFORE it is made public. This is not something that happens with carriers.
So which are they? Or can they be both?
The solution to 1984 is 1973!