The nazis were a broad popular movement. It wasn't just some dictator and his family who could be removed in a day. Most of the intellectual elite was either dead or had left the country. The nazis had replaced everyone who held any kind of official role with their own people. All the judges, university teachers, school principals, police officers, civil servants, etc, they were all nazis or opportunistic collaborators with questionable political attitudes.
There was no way all these people could have been removed from their posts or put in jail where they really belonged. Preventing them from being politically active, banning their symbols and everything they might reunite behind in order to regain power, was seen as the only practical way to diffuse a very real threat.
Also, the laws were made at a time when no law could be made without the approval the US government. After all, a large number of US, British, Russian, and other soldiers had just lost their lives liberating Europe and removing the nazis from power. It would have been insane to just let the nazis resume their political activities.
But 66 years on they are no longer a threat. They have almost no popular support, and the very specific laws that violate free speech could now be replaced by general laws against inciting violence like they exist in all countries.