You seem to image that "Supreme Law of the Land" makes the Bill of Rights into laws. Such is not the case. They govern ONLY the government.
The First Amendment starts out "Congress shall make no law...". Nowhere in the Bill of Rights is there wording that applies those rules to the People.
Do YOU have to listen to a Jehovah's Witness standing on your front porch? No. Same goes for Mormon missionaries.
Freedom of speech really only means one thing: You can speak against the government without fear of arrest. It means that the government MUST listen to your grievances in duly constituted courts.It does not mean that you can go anywhere and demand an arbitrary audience.
AFAICS, those who post to FR are agreeing to make them the owners of whatever original content is subsequently published here, making them a publisher (corrections welcome); thus, the removal or retention of such content is exclusively at their discretion, versus if they were a platform for others’ content.
The Articles and Amendments of the US Constitution are indeed law, and that is explicitly stated in Article VI; if not, they could bind no entity, whether a member of government or otherwise.