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Less well known is the paradox of tolerance: Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.

 Highlight mine . . .

I see no reason why allowing them to speak and benefit from the protections of the 1st amendment is "failing to defend a tolerant society against the onslaught of the intolerant".   Indeed, the flow of our society is in the absolute opposite direction of the intolerant.  We as a society are less tolerant (laws wize) of intolerance than we've ever been.  I see no reason that today is a special case for tearing down the 1st in the name of Popper's paradox.