I think the more pertinent reason here is that these platforms have broadcast capability (immediate communication with many people) as opposed to p2p capability (traditional SMS or phone calls). Even if Twitter was strictly chronological, without any algorithmic mutation, we'd still presumably be insisting they police content, right? I agree with your conclusion that they're publishers, but to me, what makes a publisher a publisher is not content curation or mutation, but is simply broadcast capability. And so our drive to regulate follows quite naturally from similar drives to regulate the press and media.
One involves a neutral role, in which subscribed feeds are delivered to users without modification or filtering.
The other involves an active role on the part of the platform for any number of reasons: increased engagement, removal of voices that may cause perceived damage or lack of trust in the platform itself, or other, more ideological reasons.
I've noticed an intentional avoidance of distinction, lately, between active and passive behavior on a number of fronts, from sexual activity, to medical advice and intervention, to social media publishing. It's a pretty crucial component in ethical analysis that I suspect is being intentionally blurred.
So, in your words, I would say, it is somehow fundamentally impossible for a technology to be "passive".
You may find the choices of the average person "divisive or simplistic" but the Retweet button doesn't dictate their choices.
"Policing" content, however, is all about dictating people's choices, motivated by the thought that the "police" know better than less powerful people, and reestablishing the biased filter controlled by the powerful (new-)media executives.