You're conflating a lot of things here. One is the free exchange of ideas with freedom to harass people. Another is legal versus socially accepted. A third is the difference between "the cops should be able to arrest your for X" and "I am choosing to spend my days creating a platform for X". These are all importantly different.
You're also shading over exactly who gets free speech. If digital Klansman get to freely harass black people, many of those black people will not participate in public spaces, silencing them. Indeed, that sort of ethnic cleansing is often the goal of racial abuse. See, e.g., Loewen's "Sundown Towns". So whatever "free speech absolutists" think they're up to, in practice the result is often a diminishing of the free exchange of ideas that the Founding Fathers were clearly pursuing.
You're also shading over exactly who gets free speech. If digital Klansman get to freely harass black people, many of those black people will not participate in public spaces, silencing them. Indeed, that sort of ethnic cleansing is often the goal of racial abuse. See, e.g., Loewen's "Sundown Towns". So whatever "free speech absolutists" think they're up to, in practice the result is often a diminishing of the free exchange of ideas that the Founding Fathers were clearly pursuing.