It seems like a lot of people have forgotten this fact (or failed to learn it in the first place).
I think a refresher course is in order.
Most modern people would agree that this thing called "free speech" is an imperative component of a civilized society. They believe (rightly) that freedom of speech is a human right. They also believe (wrongly) that only overtly totalitarian governments deny this right to citizens. The problem is that most modern people fail to understand that this right to speak freely must apply to every person and every idea, or else free speech is a farce, and no speech is truly protected.
The problem comes in when we begin to evaluate speech whether it might be offensive, or if its source ideology is potentially dangerous. Many, many of those same people who would answer that free speech is an inviolable human right and necessary for civil society, would change their tune if you asked them specifically about speech they find abhorrent.
Already in many "civilized" western nations, people have been penalized by the state for espousing certain ideas or using certain words. There's the guy in the UK who was fined 800 pounds for teaching a dog to do a Nazi salute, and then filming it as a joke. Others have been sentenced for more serious instances of hate speech, for questioning official historical narratives, or for calling people offensive names.
The message we are supposed to get from this is that civil society should not tolerate hate speech. And on its surface, that seems a reasonable, even noble aim. After all, millions of people have been oppressed, harmed, or murdered by bigoted mobs and political regimes based on hateful ideologies. That's certainly not something any ethical person wants to support or encourage.
But there are several problems with that line of thinking:
Speech is subjective.
Ideas, opinions, and beliefs can be interpreted different ways, depending on who is doing the reading or listening. While most of us would agree that calling for segregation or genocide is evil, we must remember that some people think that our beliefs and lifestyle choices are evil. Especially as long as humanity insists upon having government of any kind, it is extremely important in "freer" nations not to set a legal precedent for outlawing certain kinds of speech. Because you may like the people who are in charge now, and you may agree with them on which types of speech ought not to be tolerated, but it is always possible that the next ruler will be one of those who doesn't think your opinions ought to be tolerated, and the precedent will be used against you.
Offensive =/= hateful.
No matter what you say or write, chances are someone out there can find a reason to be offended by it. Doesn't matter if you meant it as a joke, if it's not offensive in your region or culture, or if the idea of it being offensive never occurred to you. If society has deemed offensive speech to be punishable speech, then we're all probably better off sewing our mouths shut.
Speech is not harmful in and of itself.
"Sticks and stones may break my bones, but words will never hurt me." Regardless of how awful a person's ideas are, they can not kill or maim you. They can't force you to do something you otherwise would not do. They can't steal from you or destroy your property. Ideas, in and of themselves, have no physical power. Of course, ideas can be used by people to foment actions that can hurt others, but even in that case it is the action, and not the idea, that is harmful. Some people like to argue that "yelling fire in a crowded theater" is a type of speech that should not be protected, but I think that is more of a fraud issue than a speech issue.
Tolerating offensive speech helps us identify our enemy.
If we don't allow assholes to be vocal about their assholery, then how can we ever expect to identify and disassociate from the assholes? When people are free to be upfront with their opinions and ideologies, others who disagree with those opinions and ideologies are also free to refuse to do business with them, to shun them, or simply to let them know that they are repugnant human beings. But when we create an environment in which assholery must be hidden behind closed doors and in private internet chats under anonymous avatars, we are denying ourselves the opportunity to disassociate from those whose beliefs we despise. Sunlight, as they say, is the best disinfectant.
Bad ideas can only be overcome by addressing them.
This is the biggest issue with refusal to tolerate abhorrent speech. No bad idea was ever disproven, no ideology-driven hate movement ever conquered, by removing it from public discourse. And in fact, banning things--even ideas--will always make them more attractive to certain personality types. The correct course of action, if you want to fight a bad idea, is to refute it relentlessly in public. In order to do that, you have to allow the people who believe in the bad idea to speak freely.
We are exposed to a wider variety of ideas than any people in history.
It kind of makes sense that some people react to the veritable smorgasbord of ideas available to them through media and the internet by demanding the silencing of ideas they dislike or disagree with. But remember that Hitler also silenced speech he disliked. It is the move of a tyrant, and any society that practices it is tyrannical.
Instead, in order to face this broad array of information, we need to thicken our skins. We need to make ourselves more and more impervious to bad ideas, and less and less easy to offend.
No speech is free unless all speech is free.