Literature is art, and art speaks to different people in different ways. This is my personal response to the short story in question.
When I read John Updike's "A & P," I don't necessarily get the sense that the story's purpose is to act as a "call to action," as it were. Although Sammy, our protagonist, takes a stand for his principles in defending the young girls who get in trouble for wearing bathing suits into his grocery store, my perception of the story's intent rests with visiting the past, and learning about ourselves through our actions.
Sammy's explanation of his behavior does not seem to encourage others to follow his example. However, to me is seems to call upon us as readers to look over the past and revisit a clarifying, pivotal moment when we took a step away from being a child, and moved toward making more adult choices and/or decisions.
Whereas the purpose of the story may seem to encourage a "walk down memory lane," moreover, I think Updike provides the reader with a vehicle that will do more than stir up memories. He asks the reader if he or she can define a similar experience, looking even farther on, then, to attempt to understand the motivations of our actions, and most importantly, how those actions have brought us to this point, today, in our lives. Who are we, how did we get here, and what have we learned from this introspective exercise?
That, I believe, is the most vital purpose of this story—or perhaps any story. In reading literature, we can learn more about who we are, and this is important in guiding us in what we choose to do and/or be in the future. At the same time, it also assists us in understanding other people as well, which I believe is equally important.