“Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience.” – C.S. Lewis
Online forums like Reddit, Twitter, Facebook, etc. are not simply "private property" in the sense addressed here. They purport themselves to be neutral public forums, and in exchange for this designation, they receive important legal protections and immunities.
Look into Section 230 of the CDA.