Sammy (and, for that matter, his coworker Stokesie) is fascinated by the girls because they are wearing bathing suits in the A&P. He is basically smitten with a girl he perceives as the leader of the group, who he nicknames, in his own mind, "Queenie." The town where he works is near a beach "colony," but far enough away that girls wearing nothing but bathing suits are a rare sight in the store. Sammy also is struck by what he perceives as the girls' dignified, almost aristocratic bearing, which stands out in comparison with the store's typical clientele, described by the narrator as housewives with their hair in curlers. Sammy is so infatuated with the girls that he makes the impulsive decision to quit when his manager, the puritanical Lengel, reprimands the girls for violating the store's dress code. Unfortunately, the girls do not seem appreciative of Sammy's chivalrous gesture, and leave him to face the consequences of his decision alone. 

In "A & P" by John Updike, why did the girls impress Sammy?

With Rebellion as a theme in Updike's "A&P," Sammie perceives the girls as mavericks who refuse to conform to the rules of dress in the store.  Enamored of Queenie, especially, Sammie reflects upon the other women who enter the store in unsightly curlers and shorts exposing their spider veins and concludes that although the girls are in swim attire, they look better than the "sheep" who are clothed; thus, to Sammie they should not be harrassed or penalized.

Sammie's consequent action of rebellion in quitting his job has more serious repercussions because he now has real problems:  He has no job and must explain to his family why he has walked out of the A & P.