Sammy might be considered an unreliable narrator in that he can only see the "visit" by the girls to his store as a totally innocent event; as a "warm-blooded" young man, he can see nothing wrong with their attire.
Sammy is very much aware of every aspect of the girls who come into the grocery store where he works. He is aware of what is covered and what is not; who is sunburned and where. His radar watches every move they make, and his description of their journey up one aisle and down another is wonderful, but certainly not objective.
It is easy to see that he is responding to the girls' beauty, confidence and state of "undress." He and the other cashier are visibly "drooling." Sammy is so overwhelmed (as a boy his age would be by three girls in bathing suits "traveling through") that he makes a mistake ringing up the woman whose order is in front of him.
When the girls are called to task by the manager, who is following the rules (as unpleasant as it makes Sammy feel), Sammy loses his head and speaks up for these girls. His manager might have let this go, in that he knows Sammy's family, but Sammy is so emotionally and physically caught up in the presence of the girls, that he "leaps on his trusty steed to be their champion," when they really don't need one, and he quits his job.
Had a guy walked in without a shirt and been approached by the manager over his inappropriate attire, Sammy would not have come to the guy's defense, and certainly would not have quit his job.
Though it may seem a noble gesture, Sammy is not thinking too straight, and is therefore, not a reliable narrator.