A useful way to think about this is that the Supreme Court ruled the state cannot punish someone for advocating genocide in the abstract at a KKK rally, not because advocating genocide is somehow a natural right or that free speech is absolute, but because the government had previously exceeded its authority by going after political dissidents (particularly Communists) using similar logic. So free speech protections in the US for hate speech are a reaction to state abuse in the context of the US.

In Europe the abuses were different so the protections are different.