Well indeed, private communities do have the right to enforce their own rules and policies in addition to public laws.
There is always a cost to speak, or write, even typing comes at a cost of the energy to manually type letters in addition to the cost of what people will think of what has been said or written.
For public realm sites meant to provide opportunity for people to be able to speak and write freely without their words being changed or censored unreasonably is what I was talking about, especially if they are government funded then they represent the tax-payers and therefore must respect their freedom of speech if they are to be considered a lawful branch of government.