Novels can sometimes overwhelm readers with a steady and growing awareness that what we see on the page is only there because of the whims of the author, who could easily have made other choices. One Hundred Years of Solitude sidesteps this pitfall by creating a world in which free will almost doesn't exist, where fate controls the actions and histories of every major character. Every prophesy and fortune-telling come to pass, and no one can escape the pull of what the cards have in store for them.
Although some characters claim to follow a moral code that ensures ethical behavior, in practice, most are able to convince themselves that whatever they want to do falls within their principles. Thus everyone comes to fulfill whatever fate has in store for them.
One of the reasons the narrator is so detached and nonjudgmental is that in a universe without free will, there can be no guilt or responsibility for any action. After all, whatever happens was fated to happen.