They have special legal obligations to, within reason, facilitate every legal transaction that comes their way. The scenarios you describe therefore cannot happen with them.
If you want internet platforms to have that same level of "anything goes", then you're arguing for them to be classified as common carriers.
I'm not sure how I feel about it though. A "commons" physical space has has an firm upper limit on how many people it can reach.
I don't think anyone is entitled to the massive amplification of their words that a large social media platform can provide without recourse to either being moderated or curated the way other broadcast media is done. A person could not perform some analog to "shouting fire in a crowded theatre" on those traditional media forms, and I don't see how doing it on the internet should change that.
The problem as always is where do you draw the line? Some free speech advocates seem to say it's too difficult so we shouldn't try. That one reasonable step could be the justification for an unreasonable one. I think that's a complete cop out. I think you do the reasonable thing, and then fight against the unreasonable thing. Not give up because it's not easy. It's a hard thing and we need to constantly as a whole and in individual circumstances assess our judgements, but not back away from making them.
“people can just send letters i fully support att not allowing group X to use their phone lines to spread info.”