Interest expense is a nonoperating expense when it is not part of a company's main operations. For example, a retailer's main operations are the purchasing and sale of merchandise, and a manufacturer's main operations are the production and sale of goods. Neither the retailer nor the manufacturer has as its main operations the borrowing and lending of money. (On the other hand, a bank's main operations involves interest expense on its depositors' savings accounts and interest revenues on its loans and bond investments.)
By reporting interest expense as a nonoperating expense, it also allows for a better comparison between the operating income of a retailer that has little debt with a retailer that has a significant amount of debt.