Why aren't hate groups banned in the US? Because the Holocaust didn't happen here. (But what does one call the slave trade and 'Indian removal'?
Hate Groups are not banned in the US, because the constitutional structure of the United States disallowed government interference of 1st Amendment speech. The 13th, 14th and 15th Amendment created inalienable freedoms to all people and did not necessitate an overhaul of the document. Concepts of Hate Speech and Groups did not exist in the late 1800s, and the Union amendment-drafters would not have voted on such limitations. They miscalculated the confederate individuals that would use the government structures to put limitations on Civil Rights.
Germany has a Hate Group ban, because they had an extremely poor constitutional structure that allowed the disintegration of checks and balances on power, allowing the government to interfere with freedom of speech and expression. Article 48 of the Weimer Constitution provided Hitler unlimited power, and consequently, the Weimer Constitution was replaced by the 1949 Constitution. There are still problems with Germany's constitutional principles, but intense world scrutiny has imposed upon the German Government a careful adherence of the equal protection principles covered by the Eternity Clause.
Imposed by the Allies who dreaded a resuscitation of Nazism in a culture that had been infected of it, and endorsed by German leaders who themselves recognized that Nazism was incompatible with any concept of democracy, whether liberal democracy in the Federal Republic or 'socialist' democracy in the DDR. On this the Soviets and the West could agree: that Germany (and Austria) must never have another Hitler. This also applies to countries that had large German minorities before the Second World War, including France (Alsace-Lorraine), Poland, Hungary, Romania, and the former Czechoslovakia -- and to such dascistic parties and causes as the Ustase in Croatia, the Iron Guard in Romania, the Arrow Cross in Hungary, Quisling's Nasjonal Samlung in Norway, the National Socialist Movement in the Netherlands, Rexists in Belgium, and Fascist Party of Italy. In the Far East, the political umbrella of the single-Party that ruled Japan during World War II was also outlawed.
They never will be missed.
Had history been inverted, with a democratic Germany and a somewhat-democratic Japan defeating a KKK-dominated America that committed Holocaust-like atrocities, then America would have bans on hate groups and their visual and verbal signals.
