Sammy refers to the regular customers in the A&P as sheep because to him, they are like a herd of sheep. They are timid and do what they are told. They flock, to his mind, into one indiscriminate herd. Calling them sheep shows contempt and helps Sammy make a distinction between them and the teenage girl he calls Queenie and her two companions. Sammy observes that

The sheep pushing their carts down the aisle—the girls were walking against the usual traffic (not that we have one-way signs or anything)—were pretty hilarious. You could see them, when Queenie's white shoulders dawned on them, kind of jerk, or hop, or hiccup, but their eyes snapped back to their own baskets and on they pushed.

The "sheep" in the quote above are a contrast to Queenie. She stands out, but they avoid trouble and mind their own business, even though it is clear they notice with surprise—a "jerk, or hop, or hiccup"—that the girls are dressed inappropriately for the store.

A short time later, Lengel, the manager, tells Queenie that she and the other girls have to come into the store with their shoulders covered, as this is store policy. Queenie argues back, but Lengel insists it is the policy. At this point, Sammy notes that the other customers, sensing a scene, stay away from the girls and all move together into the cash register line run by Sammy's friend Stokesie. Sammy once again contrasts Queenie's bold behavior to the compliant behavior of the regular customers:

All this while, the customers had been showing up with their carts but, you know, sheep, seeing a scene, they had all bunched up on Stokesie.

At this point, Sammy feels he has to make a decision, either to be one of the sheep or side with the girls, whom he admires. He knows he is one of the sheep. He, too, is a working class townie who does what he is told, not an affluent summer vacationer. However, he rebels against a life of being a sheep. He quits in solidarity with the girls and walks out. Once outside, he notes,

I could see Lengel in my place in the slot, checking the sheep through.

As he watches, Sammy realizes that his life has suddenly gotten much harder: he has made a bold decision, but it is easier to go through life as a sheep.

In the short story "A & P" by John Updike, Sammy thinks of the customers as sheep because they all appear alike to him, and they seem mindless. Just as sheep will flock and follow one another, so do the customers at the A & P as they go up and down the aisles, blindly reading their shopping lists.

This is different from the girls who enter the store in their bathing attire and walk the wrong way from the sheep, up and down the aisles. Sammy says, "The sheep pushing their carts down the aisle—the girls were walking against the usual traffic (not that we have one way signs or anything)..."(para 5). The girls, therefore, illustrate the unconventional, going against the rules.

Later on in the story, Sammy refers to the customers once again as sheep as they are "all bunched up" when the girls are being reprimanded by Lengel. Again, this illustrates their similar, herd-like behavior. However, when Sammy refers to them as sheep, it also shows that he thinks that the girls are better than the other customers. This also illustrates that he, too, thinks himself better than the sheep. When Sammy quickly quits his job, he also acts in an unconventional manner.

The shoppers in the store are like sheep to Sammy because they blindly go up and down the aisles and then head to the checkout counters just as sheep are blindly herded to slaughter through chutes. When Lengel embarrasses the three girls for dressing inappropriately in the store, the others all crowd together nervously like scared sheep. Only Sammy dares to defy the policy of the store and society and challenge the rules. He's the only one who is willing to say society's rules, written and unwritten, are not always fair. Even the three girls in bathing suits give in to society's mores as they leave the store humiliated by Lengel. Sammy quits because he sees Lengel and the shoppers as blindly following the protocol of society, and yet no one seems to feel that Lengel's embarrassment of the girls is wrong. Sammy challenges whether Lengel should have the right to treat others in this fashion.

Sammy thinks of customers as sheep because he really doesn't care about them as individuals.  They are all the same to him...simply people that are there to buy goods.  Also, let's think about how sheep are regarded and taken care of.  They are not regarded as the smartest animals, first of all.  Also, they are herded through gates by Australian cattle dogs or other types of herding dogs, just as the customers are herded through the check out lines at the A & P store where Sammy works.  The reference of people as sheep is not a pleasant or flattering one, then, as we can see!