Banks currently can charge a fee when customers make a purchase with a debit card where the cost of the purchase exceeds the customers available balance. The banks know the customer does not have sufficient funds to cover the cost, but they approve the transaction anyway, and then charge the customer an exorbitant fee.
The correct way to handle the transaction would be to simply deny the transaction.
Alternatively, if the customer opts-in to receive overdraft protection, then the only option should be to be have the bank take funds from a secondary account. If funds in the secondary account are insufficient, then deny the transaction. Simple. No Fees.
It's counter-productive to charge a fee to a customer who obviously does not have the funds to cover the charge -- or the fee.