Bryan Caplan reminds us of a great old puzzle: why are libel, slander, and blackmail illegal? Bryan and I find it easier to understand two extreme positions than the actual intermediate mixture we have. The extremes:
Punish Falsehood – Authorities monitor what people say and punish them for saying things authorities believe false. Of course authorities pay attention to transaction costs; it isn't feasible to react to every little falsehood. But if authorities believe something, well they believe it to be true, and so they usually expect people to be harmed by believing the opposite. When a falsehood is important enough, punish it.
Listener Beware – It is up to listeners to decide what to believe. Speakers who are eager to be believed can, if they choose, subject themselves to penalties if they can be proved wrong. Such "fraud" penalties can even be in contracts. But since listeners can choose to ignore speakers they don't respect, and can use any basis they think appropriate to decide who to believe, it is not clear why we should risk empowering authorities to intervene further. If you hear something you think false, just say so.
Actual policy is an odd mix of these extremes. People are free to make any strange religious or political claims, but are not free to make medical claims, claims about people, or claims at trials. Nor are folks free to be paid not to tell truths. Surely there are some implicit bias theories behind these rules, but until these theories are made more explicit it remains hard to evaluate how much sense these rules make.
Added: In the US, alcohol companies may not buy TV ads truthfully saying most studies find people who drink more are healthier, and trial witnesses may not truthfully tell rumors they've heard about the accused. Most comments so far basically repeat standard arguments for one or two of the extreme positions; the puzzle is how to tell when one or the other is most appropriate.
More Added: Bryan and I want to debate; tell us what to debate about.
Institutions almost always punish falsehood although the manner differs. Religion punishes through doctrinal councils and excommunication, politics through parties and elections, science through peer review and rejection, the law through courts and penalties, markets through law and judgments, though there exist unsettled areas within them where listener beware operates until settled. Only inter institutional and interpersonal relations open themselves to arbitration in the absence of which listener beware rules.
I strongly agree with UlrichRoarke: this post (including the comments) has lots and lots of bad law.
It's a good thing it's not illegal to say untrue things about the law in a blog post, or a lot of people participating in this thread would be in trouble.
For instance, a commenter says: "in the USA factually true statements, as well as statements of personal opinion, are essentially immune." No. Truth is a defense to libel and slander, but there are other torts you can be liable for even based on making true statements about someone. If I procure your medical records and publish them in a newsletter, I may well have probably broken the law; you'd need to research the applicable state tort law.
More fundamentally: the post itself is assuming the law must be based on "bias." ("Surely there are some implicit bias theories behind these rules...") You're saying that because that's the theme of this blog. But there are innumerable pages worth of judicial reasoning in American case law explaining why we draw the lines where we do. Now, that reasoning may be open to criticism -- I'm not saying we have to go along with it. But why not give one single example of a Supreme Court case (or a law review article, etc.) on a specific 1st Amendment issue? There's been so much written about why you give special protection to political speech, why it's OK to heavily regulate commercial speech, etc.
When the blogger says, "until these theories are made more explicit it remains hard to evaluate how much sense these rules make," I wonder: "more explicit" THAN WHAT? Do you realize that when a 1st Amendment issue gets decided, it's frequently accompanied by a fairly nuanced written opinion going through the relevant concerns, often at great length? And even if you think judicial opinions are too focused on specific legal issues (though they can include lots of tangential material), what about scholarly articles, treatises, and law school casebooks? Have you seriously examined these before concluding that the theories underlying the regulation of speech needs to be "more explicit"?
Again, I'm not endorsing everything those theorists have written -- they disagree amongst each other sometimes, so they're not always right. But a more illuminating post would pick out something they've written and explain what it gets wrong, not simply assume that no one has given an adequate explanation.