You are correct in your assessment that Hersey's tone and style are primarily journalistic in this novel. This is evidenced by his weaving of the six accounts of six different but equally important characters who seem to have nothing to do with one another outside of the experience of a common tragedy. Each character's story is presented in short segments, and not necessarily in chronological nor any uniform order. It is as if the author has interviewed each character and presents their separate and distant experiences in a simultaneous time line. Additionally, Hersey employs the use of flashback in order to provide background information without taking away from the action of the story.
There are a few theories as to the purpose for this particular style. First, this story is certainly fiction, but it is based on a very real historic event. Some suggest that Hersey wished to portray as realistic of a story as possible so that his audience would emotionally connect to the common human experience of suffering, survival, and hope that was a reality for many who actually lived through the Hiroshima bombing. It has also been suggested that the objective journalistic tone allows the author to remain as "truthful" as possible and the story to be presented with very little bias. This of course must take into account, again, the fact that the story itself is fiction. Though the characters are not real, one purpose behind this style could be to make them seem as realistic as possible. Additionally, this allows the reader to react emotionally with very little author influence or contrived emotion. Finally, the presentation of the six stories in short segments makes the story suspenseful and dramatic, but heightens the characters over the events of the bombing. Rather than write a story about a tragic event which focuses mostly on the event itself, it is clear Hersey wishes to heighten the humanity of those affected.