It’s more a result of Americans respect for law and order; and even a Klansman is almost always assured and afraid that any violence will lead to harsh legal penalties being imposed upon himself. (This is demonstrable by the fact that the Klan’s #1 enemy are blacks. And there are plenty of them in the South. Yet race-based physical violence against them is also very rare. [In the few cases it does happen, it always becomes a major news story.]) Unlike in Europe, as described above by a resident there, where not only isn’t such violence rare, but it is done publicly in major European cities.
Which is all a testament in favor of America.