First off, when discussing free speech, it is necessary to set aside a couple of ground rules.
1) No one is suggesting that the right to say something requires people accord that speech with agreement or enjoyment nor that it precludes the capacity to be offended or annoyed by it. It is equally available to other people to take to their own faculties to complain, cajole, argue, and even ignore the exercise of speech that they find disagreeable.
2) What is being said by the notion of "free speech" is that the government should not punish or preoccupy its limited time and energy and resources by policing the content of speech, whether to discern its offensiveness or quality. What we are talking about is the freedom from government sanctions or persecution to say absurd, offensive, or enlightening things. Not the freedom from any consequence to speech that offends or is absurd. Social consequences of derision, condemnation, or exclusion from participation in "polite" company and conversation are entirely natural and in many cases, are themselves exercises in free speech.
Both of these imply however that should someone say something deeply offensive, and it does not fall into a category of unprotected speech such as a direct and imminent incitement of violence toward or threat to bodily harm of other persons, this entails the speaker a right to the same protections of their person and property as anyone else. No matter how offensive someone is in their speech, that doesn't give people the right to assault, to threaten, or to attempt to kill them, or to deface or destroy businesses or homes of that person (boycotting a business or economically sanctioning it is entirely sensible). Even if that may be a common reaction to the nature of people's insulting or offensive public and private deliberations.
When the subject of offensive hate speech surfaces, this is a category of speech that may be considered identifiable in its contents, typically condemning a group of other persons or subjecting individuals to offensive language and attitudes on the basis of their prospective membership in a group of unpopular persons. What is being argued is not that such speech should be roundly supported by endorsing its content, but that the suppression of such speech by governments is an inherently dangerous and arbitrary power to grant governments the ability to do. That it will rarely or at least highly inconsistently be applied in a rigorous way to decide the fate of offensive and hateful people in a manner that accords with the people who wish it to be so that these powers should exist.
Well-meaning people wishing to provide a force of law and legal sanction to their social opprobrium should recognize a couple of things. First that governments are not composed largely of "well-meaning" people who necessarily agree on the nature of what constitutes offensive speech. Societies are not, so it should not be surprising that democratically elected governments provide a vast array of distinctions on what is and is not offensive or otherwise to be considered dangerous. Speech that suggests support for say, a terrorist organization and its causes, is regulated in part by what governments decide constitutes a terrorist organization as this is defined arbitrarily and politically rather than by reference to any national or international standards (which itself could be arbitrarily and politically determined). Speech that suggests offense by considering difficult empirical questions that may have implications on our strongly held religious or cultural ideologies can likewise vary widely on these policy grounds based on which religious or cultural bodies are well-organized and politically engaged enough to provide cover for their beliefs and ideals to be free of challenge. Second they should be aware that the premise of freedom of speech permits and tacitly endorses a means to push back against speech that is hateful or offensive. That is that people are already provided a power and method to raise awareness that they are under social attack via the devices of speech and expression.
The basis point for presenting hate speech as a type of regulated activity by the government is that these actions are being privately and commonly used against marginal or unpopular groups (typically ethnic or religious minorities), and thus that public social forces alone are insufficient to push back against such well-entrenched biases. As above however, it is not clear that governments will always be able to devise appropriate targets for protection. Indeed, if these are unpopular groups, it is likely that providing an avenue for political protection is just as liable to end up with political maneuvers that target rather than defend in the long run as other groups may petition for protection, and being well-organized may be more likely to attain it. Any number of policing strategies, such as stop and frisk or the NYPD's extensive surveillance of Muslims to the existence of a variety of racial codes in the Jim Crow South or South Africa, should be a warning that the passage of laws is often used as a weapon to continue and extend or entrench further marginalization by amplifying pre-existing biases, not to oppose oppression.
The biggest reason behind not placing the government in charge of making these determinations is that it will shift over time what is considered offensive, or hateful, or otherwise in need of censorship. Societies which are free to make these determinations privately already can decide that a word or a type of expression is no longer in need of careful censorship for all and that those who are offended can make efforts to police or combat such speech on their own rather than require the rest of us to do likewise and thereby paternalise our consumer choices for content and expression. The US in some spheres of society does not have this ability. Television and radio are more regulated to avoid "certain words" or certain content (usually relating to nudity or sex) where instead they should be more monitored the way cable television or to a certain degree movies or music are. There possibly offensive speech or content is labeled voluntarily by the production of that content so as to be avoided by those who deem it necessary to do so, or consumed by those who wish to see it.
What we also find is that those who control the power of the censor will also change who they are in a democratic society. That is elections and appointments (and associated terms of office) will amend what persons select what offenses will be controlled if this power is available. They will have their own opinions and offenses that they will seek out and this will be at odds with by changing or not changing with these alterations in the demands for censorship by the public at large. Providing this power does not guarantee that some particular group's acknowledgements of offensive material will be deemed offensive enough to constrain with laws, or that it will remain so, or that material that is no longer considered offensive will not remain punishable. Leaving it out of the hands of laws to decide what is offensive allows social functions to perform that action and react to all of these changes while protecting people from direct action (or violent reprisal).
If speech is, as in the Paris case, considered blasphemous and insulting to a religious group, that is not the affair of society to say that we should either restrict or promote such speech by using laws. We can indicate our displeasure with the contents. Or promote it if we are pleased by it. And we can argue for the utility of such expressions in challenging religious beliefs or against them in the manner of being against sacred textual laws and thus "offenses against god" (whichever god that was, or how a god is offended by possibly humorous expressions is not generally explained). These are not offenses against legal codes however, nor should they be implied to permit or require religious reprisal (or sanction). Nor are they universally held to be offensive at all times throughout the religion's history or across all of its cultural and ideological adherents anyway that we should regard the offense of an agitated few as a basis point for censorship at large, as we are incapable of providing clear guidelines of what will be out of bounds and what is accepted to frame this as a legal question.
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