> They are _very_ clearly acting as publishers as well.
Then they should be held liable for every post/tweet that goes on their platform! Will they be willing to accept that liability like the media houses do? You can sue the media house if it posts something that is false. You can't sue Facebook for a post someone put on it. Facebook will call itself a platform and deny taking liability for someone's post.
> It does not make sense to try to argue that Twitter is either a platform or a publisher and nothing in between
It does make sense. Your nature of business defines the regulations that apply on you. I can't start a software business today and 3 years later arbitrarily decide to convert it to a defense manufacturing company and still hope to retain the same regulatory rules that apply for software companies. There are regulatory policies for every company based on its nature of business. You are creating a monster if you allow these social media companies to have all sorts of policies without any regulatory oversight!
EDIT:
> You see a specific-as-possible individual experience published just for you.
Yes and it still is a platform because the functionality (personal recommendations) is built into the platform as a feature right? It is a AI tool that recommends a feed based on your likes and dislikes. Is this the same as publishing? Nope. With publishing you have a process where posts are edited, verified and then shown to the public. There is some notion of "trust". So when I watch news I know that the news channel has not just curated, but also verified and then is showing me the content that I can then consume. That verification step is essential and is what separates a "Publisher" from a "Platform". Twitter never "verified" the feed right? It just curated it. As a platform it can only "moderate" based on its policies that prohibit certain acts that are illegal by law: like sharing child pornography or calling for violence or gambling. This is required by law and that is where moderation tools come into play. Twitter, by censoring/editing/attaching information to Trump's tweet is behaving as a "publisher" by actually verifying the content of the tweet in question. It does not fall under "moderation" as there is nothing there that is illegal for them to moderate. The feed I consume through personal recommendations is curated but still unverified. I still have to figure out for myself if the post/tweet in question is correct or incorrect. The onus of verification rests with me, the user. That is why it is a platform. The onus of verification in case of media channels rest with the channel. Because they are operating on the notion of "trust". That as a consumer of their content I have trust in what they show me. I don't need to verify. That trust is broken if they start lying. They can lie and spread fake news. But they will be held liable for it. That is why they are a publisher and have to go through the headache of verifying the news before it is broadcast to the public.
Twitter is like a post office. You send mails via post, you expect the postman to deliver the post to the nearest post office, which sorts and aggregates those posts and then pushes those posts to the relevant po box for dispatch. Imagine if the post office started to open the mails and tamper with the messaging. Would you be okay with it? The post office can only open posts if they find something criminal and that also has to be done in a systematic manner: An authority should be present while the post is being opened, every step is to be recorded, the items within the post should be carefully handled and recorded all the way up until the post is sealed. And then the authority must sign off by saying that the post did not contain anything bad (like drugs/weapons whatever that is declared illegal in that jurisdiction). That is the only powers a post office has.
Now imagine if the post office started messing with the posts. Imagine if you were running for election and you distribute flyers through postal mail. One of the employees in the post office doesn't agree with your ideology and decides to tamper with the flyer. Instead of modifying the flyer he attaches a sticky note with his own comments so whoever reads the flyer will also read the sticky note. Would this not be considered tampering with the messaging? How is Twitter attaching a notice to Trump's tweet any different from this? Only difference is that it is virtual and not physical?