Free speech online doesn't even seem to be a particularly well-defined concept. Those who extol it the loudest are often looking to have the millionth "good faith discussion" about The Bell Curve, or use slurs as "just a joke", or promote a "dating and lifestyle coaching" business to teenage boys. If all they want is carte-blanche to say absolutely anything without being censored, I guess they only need to spin up a web server of their own, or run a lemmy instance. But what they actually want is to bypass the moderation rules on widely-used platforms and shit on the social contract. It's the same reason they don't show pornography, snuff footage, or other damaging content on television.