The (also legitimate) argument is that freedom of speech should extend to privately-owned de facto commons, and is a concept beyond the enumeration in the American Bill of Rights.
However, for laws to be created that force them to publish posts that fall under "free speech" their algorithms must be fully open and auditable. Otherwise they still hold the power to sensor, or their algorithms could be covertly gamed by those in the know.
Otherwise your speech might not be the same, or as free as mine, but who would know?
But all companies seek to become monopolies for purposes of profit, while avoiding being seen as monopolies for purposes of responsibility.