We have holocaust deniers and neo-Nazis over here in the US, too. They're considered fringe nutjobs--because they air their views in public, speak freely, and everyone can see for themselves what idiots they are. When you ban that kind of nonsense, you drive it underground--its adherents easily convince themselves they are a persecuted minority speaking Truth to Power, or otherwise onto something real and important, because they are threatening enough to the powers-that-be to be banned. That makes them more attractive to those who are disaffected and not used to dealing with fringe nonsense because it's all a big, underground secret.
Secrecy encourages this kind of nonsense; repeated public exposure reveals it in all its stupidity and vileness.
Also, we have a large category of people who trade in Nazi paraphenalia in the U.S. that have nothing to do with neo-Nazis; they are people who collect WWII and other historical memorabilia. Not everyone who has an SS dress dagger in their desk is a secret Nazi; more often, they or their parents fought in WWII and it's a part of their history. I see what Germany does as trying to suppress history, and you know what is said about those who forget history...
The Communist Party was banned in the U.S. because it advocated violent overthrow of the US government. Notice that the various Socialist parties over here, who advocate "let's get elected and change the laws legally", are perfectly legal.