The food worker on break may drink from an uncovered cup in the kitchen.
To begin with, the people who come to dine in wouldn't be exactly happy to witness the food worker to drink water in the deli area or the buffet area or in the dining room. Those would be the places where the water would be served or placed for the people who come to eat. It is a safe mannerism if the food worker drinks water from the kitchen as it is more appropriate and right thing to do. As basic rules, a dining place would know how to keep the place hygienic and clean and kitchen would be the place where the uncovered cup would be kept clean, especially for the workers' sake.