Various incarnations of what is known as "Godwin's Law" suggest that if a discussion goes on long enough, it will invariably devolve into one party comparing the other party's position to Hitler or the Nazis. Similar principles include the argumentum ad Hitlerum, which is a fallacious argument that attempts to discredit another's position by invoking Hitler.
On a family vacation, I once saw a fascinating History Channel special on the American Nazi movement in the late twentieth century. The special piqued my interest for several reasons. First, I have done research on the relationship between racist violence, economic development, and social change. Second, the power of ideas--particularly divisive ideas that pit a wise and virtuous "us" versus a degraded and inferior "them"--manifests itself in historical atrocities like the Holocaust, Jim Crow, the Great Leap Forward, and the Gulag Archipelago. Third, people sometimes conflate the right to speak freely with the right to an audience and a subsidized soapbox. Conflicts about "free speech" for groups like the Nazis and the KKK are rooted not in the merits (or lack thereof) of hate speech but in questions about who gets to use public property.
History provides ample evidence that ideas matter, and not just the ideas of those who stand to gain or lose materially from policies. Southern slavery was undergirded by a racist ideology that developed as part of the system and that helped ensure support for the system even among non-slaveholders, and the Nazi atrocities of the 1930s and 1940s were enabled in part by anti-Semitic public opinion. It has been said that eternal vigilance is the price of liberty, and the power of racism throughout history provides ample evidence of cases in which opinion leaders were asleep at the wheel. How much poorer are we for it? Fortunately, great thinkers like Ludwig von Mises and others escaped the Nazis. How many great minds perished in the concentration camps? What great books were never written and what great advances in science were never made because racist violence felled people in their prime? We will never know, but it is a question that deserves somber reflection.
Public property creates many thorny issues about freedom of speech that can only be resolved with private property. The aforementioned History Channel special discussed a conflict in which the American Nazi party was trying to get the permits it wanted in order to stage a rally. With the attempted extermination of the Jews during the Holocaust only a few decades old, this was still an extremely divisive issue. The root of the conflict, however, concerned the use of taxpayer funded private lands. While local authorities tried to stop the Nazis at every turn, they were ultimately represented by the ACLU (and by a Jewish lawyer, incidentally) in a case in which they were ultimately granted the right to march.
You have a clear right to express views that others find offensive, but airing offensive views raises the prospect of violent objection from those who are offended. Who is responsible for paying for security? It certainly isn't the taxpayer or the municipal authorities. Further, the police-perpetrated outrages that occurred during the Civil Rights Era suggested that it is probably a mistake to trust the government to keep the peace.
Spewing hate would have been much more difficult in a world with secure private property rights. Right now, expressive racism is cheap. You need to be able to fill out the forms to get a permit, carry a few signs, shout loudly, and if you are really sharp, you might need someone who can write a good press release. My guess is that hate speech would be much more expensive if now-public spaces were privately owned. You would not need a permit, but you would need to rent a space from a private owner. This is not a praxeological certainty, but I would expect that businesses who earn a reputation for supporting Nazis or the KKK would lose business.
There is no principled way to exclude people with hateful messages from use of the public square. There are, however, principled ways to exclude people with hateful messages from the use of private property. I agree with Steven Landsburg that tolerance and pluralism are the prices we pay for freedom, but we can draw the line at the use of real resources. There are a lot of things I can do with my time and money that I would greatly prefer to helping foot the bill for security and port-a-potties for KKK rallies. I do not object to racists' rights to spread their message, though I object to their message. I especially object to their use of others' money to spread that message. If you wish to spew hate, I'm not going to stop you. I'm also not going to volunteer to provide you with a forum or security, either.
Art Carden is Associate Professor of Economics at Samford University's Brock School of Business, and he is by his own admission as Koched up as they come: he has an award named for Charles G. Koch in his office, he does a lot of work for and is affiliated with an array of Koch-related organizations, and he has applied for and received money from the Charles Koch Foundation to host on-campus events.