These social media platforms don’t act like common carriers.
It’s a fundamental necessity of the business that social media platforms be able to deliver enough good content that users want to join and engage while filtering out enough bad content that users don’t get fed up and leave.
So they implement all kinds of mechanisms designed to promote and emphasize “good” content and minimize “bad” content. It’s important to understand that “good” means good for business and “bad” means bad for business, and of course there are sometimes short-term vs long-term considerations to balance.
Mechanisms like search rank algorithms, retweets, comments, likes, content policies, moderation, flags, bans, etc.
If the government takes some of these mechanisms away from a social media platform they will (1) fundamentally change the nature of the social media platform — quite possibly breaking what made it a desirable place to share in the first place; (2) prevent these companies from making business decisions in their own business interests.
Imagine HN with no moderation, or your favorite narrow-interest subreddits full of spam and political posts with no one allowed to moderate them or ban the crap posters. It entirely defeats the purpose.
Is this really what we want?
Also, remember that platforms much more permissive than the big social media platforms exist that are accessible over the same internet, not to mention plain old web sites. So what exactly is the need to force, say Facebook or Twitter to have very permissive content policies?
I think what many people concerned with this actually want is for social media platforms to continue to moderate and filter bad content and promote good content, but they want to tell the platform what “good” and “bad” mean according to their own opinions rather than letting them determine that for themselves.