The written and unwritten rules we as a society establish for our places are usually the limitation of some freedom to protect another. You are not allowed to scream loudly in a library, because then one persons freedom to scream would destroy the freedom of all other visitors to read in peace. Collectively we agreed that we gain freedom by sacrificing certain unfitting behavior. This must not even be rational, it is pure convention by the majority using a place (e.g. think about how the rules in various houses of worship work).

Of course one could now argue that you can forbid behavior for arbitrary reasons, without making such a deliberate tradeoff. After all, exactly this is an integral part of authotarian structures all around the world. However in many online places the kind of freedom people argue for is rarely the freedom to eloquently postulate a controversial opinion. More often it is being an asshole to others. And just like in the library example this can and will affect the freedoms of others. Every screaming troll makes a good, productive debate more unlikely, which is why nearly every place where decent online debate happens, bans this kind of behaviour.

And this is good. Different places serve different purposes and have therefore their own written and unwritten rules. This in itself is not a problem at all.

Don't get me wrong here, but if someone would complain at the swimming pool that we are not allowed to pee into it and are therefore not free, I don't question the pool rules, I question why they want to do that in the first place.

If the rules say "no swimming" this would be something different altogether.