Shocking?
I think the idea that this movie was "shocking" to the American public is Hollywood hyperbole. The country was less than a decade removed from WWII, and millions of Americans had witnessed the inhumanity of war. They had also grown up watching gangster movies, reading about real-life gangsters like Dillinger and Capone, and watching westerns with bad guys that made the bikers in this movie look like boy scouts. Americans were not unfamiliar with bad behavior, and the concept of gangs terrorizing people was hardly ground breaking.
This was really just a western where the gangs rode motorcycles instead of horses. The big difference was that there was no real hero in this movie that saves the day. It was a rather amoral tale, and I think that fact probably annoyed more people than it actually "shocked".