Free speech isn't the First Amendment. Free speech is a broad foundational principle of liberalism, and the First Amendment is just an encoding of this principle in the context of the U.S. government. But go back to Mill's "On Liberty" and you'll find that he was just as concerned about threats to free speech stemming from social disapprobation as those from the government.
Anyway, I prefer this modified version of the strip: https://pbs.twimg.com/media/ECqxDQGVAAAXUgK?format=jpg&name=...