Banks and credit unions have long pitched debit cards as a convenient and prudent way to buy. But a growing number are now allowing consumers to exceed their balances — for a hefty, some say, abusive, fee. Banks market it as overdraft protection, and the fees it generates have become an important stream of revenue fees for the banking industry. This year alone, banks are expected to bring in $27 billion by covering overdrafts on checking accounts, typically on debit card purchases or checks that exceed a customer’s balance.