Burning a Koran is front of a mosque is a deliberate incitement to violence. The US allows far more freedom of speech than most countries. Germany, for example, bans any favorable reference to Nazism. Even here in the US, there is not complete freedom of speech. You cannot threaten violence against someone or advocate the violent overthrow of the government.
While we should always err in favor of free speech, there are and should be limits. Yes, this can be a long, slippery slope to repression and needs to be guarded against. But Terry Jones’ previous Koran burning did lead to people being killed. He should not be permitted to do it in public and forthe same reason that the KKK cannot burn crosses on public property.