WESTHAMPTON BEACH—The Beach Bakery Café here is popular for its cupcakes and bialys alike.

That the café is kosher goes unnoticed by most. "Nothing changed in this bakery when it became kosher," said Avraham Bronstein, the 30-year-old assistant rabbi of the nearby Hampton Synagogue, who scrutinizes the goods here every few days. "And nothing's going to change with an eruv," a symbolic boundary that allows Orthodox Jews to conduct certain activities otherwise prohibited during the Sabbath, such as pushing strollers or carrying...