In "A&P," why does Sammy make the decision that he does at the end of the story?

In this famous short story by John Updike, a brief encounter in a supermarket becomes a defining "coming of age" moment for a teenage boy named Sammy. One day in summer, nineteen year old Sammy is working when three teenage girls come into the store. They are scantily dressed, and the store manager berates them for it and tells them they need to leave. Sammy is somewhat horrified by this treatment of the girls, and as they are leaving he loudly announces "I quit" to his boss. He makes the decision to do this rather hastily, partly out of a desire to impress the girls, possibly to make himself into a sort of hero or helper to them, to acknowledge their humiliation and provide them a show of support.
On some level, this seems like a rather sophomoric thing to do, prompted by a teenage boy's romantic notion of chivalry, or even a form of offhanded flirtation, perhaps. But the story describes Sammy's actions as being loaded with potential consequences, having repercussions for his future. This moment is full of lessons that the future Sammy may look back on with regret or annoyance. His impulsive decision to quit his job in protest of the girls' treatment feels to him like a noble act. But instead of quitting, what if he had spoken up and challenged his boss, stating why he disagreed with his actions? By announcing that he is quitting, Sammy chooses a non-confrontational way of showing his displeasure; once he quits, he can walk out of the store and never confront his boss again. But actually confronting the boss's speech and behavior meant risking an escalation of the boss's anger; it may have even ended with Sammy being fired. By quitting, Sammy exerts a certain amount of control over the situation; but at what cost? It is as if this first test of his "manhood" came unbidden, and he wasn't quite ready.