This argument is touted around a lot to justify censorship by private parties. Don't be misled - it is censorship. The law doesn't protect this kind of speech, which means they are legally permitted to participate in censorship, but it is censorship all the same. Yours is a common and poor argument for justifying this behavior. There are plenty of things which are both legal and immoral. I would prefer to participate in a transparently moderated community where the rules are clearly defined and aren't selectively enforced. This makes for a better community.
They don't have a legal obligation to behave this way, but that needs to stop being used as an excuse to shut down discussions on how to make the community better.
They don't have a legal obligation to behave this way, but that needs to stop being used as an excuse to shut down discussions on how to make the community better.