The reason for this is that the book is set during the Great Depression.
When Franklin D. Roosevelt became president, he gave a speech that was meant to reassure Americans that things were going to get better. In that speech, the most famous line was that "we have nothing to fear but fear itself."
So when Scout says this in Chapter 1, it's not that Maycomb County in particular has nothing to fear. What she is saying is just that some in Maycomb are feeling more optimistic because of what the President said to the nation in general.
I agreed with the first editor but I also see this statement as foreshadowing. The south was a very difficult place during the time of Scout's youth. People were afraid of so many things. It is human nature to be afraid of what they don't know (a statement which Atticus restates later in the story).
Predjudice stems from the fear of not knowing another group or race of people. By not knowing them one is able to create dynamics that can be very frightening. This is also reflected in the way the townspeople have turned the "invisible" Boo Radley into the town boogie man. Since no one sees him they create an image of who he is and what he looks like.
The white townspeople's perception of black people was no different. They were not involved in the day to day habits, worries, concerns, loves, or laughter of the black people in the town so they feared what they did not know and created their own stories around what black people, especially black men were like.