People join specific social groups because of conversational bias. It might be a mums group where the content is mainly baby related. Or a theatre group with talks about fine arts. Or a retro gaming group that talks about old computer games. They will have policies in place to ensure the content of their group stays focused. Some groups will be aimed at families or being safe for work so might have a no nudity rule. Some might have a no trolling rule because they want a friendly atmosphere. Some might have a no sales rule because they don’t want their group to turn into yet another market place. These are all TOS, censorship rules if you will, that are placed in specialist groups.

Your reasoning would say I could join a kids cartoon group and post extreme pornography because it’s legal and if any parents object then they should have the law changed to ban porn for everyone.

Quite obviously that’s a dumb way of managing online content. Let the platforms manage what content they deem appropriate for their specific audiences and if you really want a zero-censorship community then you personally should join one rather than forcing every man, woman and child into wading through the same content you personally enjoy.

They are a business, how can you mandate that a private business has to host "everything".

The same way we regulate any business and mandate they abide by regulation. Or the same way we force phone companies to be regulated as common carriers. The same way your power utility cannot cut you off based on your political opinion. All it takes is political will.

Title II Common Carrier regulations.

I'm not entirely sure that's the best model for today, but that's the historic model for a private business that's compelled to allow all legal speech. That's why UPS and FedEx still have to deliver to the New Order (the current incarnation of the American Nazi party).