I think it's OK to say (1) you are allowed to send pornographic magazines through the mail from your place of business, but (2) you are not allowed to put a pornographic image on the front of your building.
I also think it's fair to carry this analogy over to the internet, and the difference between an email and a social media post.
A social media post is read only by the people who subscribed to the author, and those who followed the links to it. Not quite the same as a public building that random people walk past, and have to see regardless of whether they want to or not.
And that would be fine, social media companies don't have to promote "objectionable" content in curated public feeds, but people who deliberately subscribed to or are looking for that content should still be able to see it (like Trump's feed). That's the step people are objecting to, so the analogy you draw just isn't relevant.
I also think it's fair to carry this analogy over to the internet, and the difference between an email and a social media post.