One universal theme is individuality and free will. The girls have to know that they will draw attention by walking into the store in just their bathing suits. In the context of this story and its time, I would say that their exhibition is sexually provocative; but not so much as the more modern sense of self-objectification (as in flaunting promiscuity as the "dumb blonde.") I think it is more about freedom of expression in general in a 1960s anti-conventionalism. Yeah, they're young, but they're aware and I think their actions might be seen as juvenile and making a sort of adolescent anti-traditional statement.
Sammy, in quiting his job, asserts his free will and individuality, knowing that this decision will follow him. It may be harder for him to get a job with this on his record. Like the duality of the girls, who go into the store in bathing suits just to be noticed and to spite conservatism, Sammy quits to assert his individuality and free will and to impress or stand up for the girls, who don't seem to notice.