The g&m is not preventing you from expressing yourself.  It is the same as your neighbour removing a sign you put on his lawn to protest or sell something.  Your rights have not been violated, you just can't do it anywhere you please.  We have specific laws that address the times when you CAN do things on someone else's private property (ie strikers on a picket line).   This is not such a case.
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I relied on the ACLU's description in "what is censorship".   The wiki definition used by grub and the oxford dictionary both are not clear on when censorship occurs outside gov't.  ACLU has specific examples of private censorship.  No sources cite a newspaper comment section as a form of censorship.  Because it is not.