A sign at Naidre's, a small neighborhood coffee shop in Brooklyn, N.Y., begins warmly: "Dear customers, we are absolutely thrilled that you like us so much that you want to spend the day..."

But, it continues, "...people gotta eat, and to eat they gotta sit." At Naidre's in Park Slope and its second location in nearby Carroll Gardens, Wi-Fi is free. But since the spring of 2008, no laptops have been allowed between 11 a.m. and 2 p.m. weekdays and 10 a.m. and 3 p.m. weekends, unless the customer is eating and typing at the...