>I can guess the answer - people (hopefully) using the retailer to get cash withdrawals from their debit cards, or (hopefully not) getting cash advances on their credit cards. Buy a 99c pack of gum, ask the cashier to make it $100.99 on their card of choice.
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>Speaking of which, debit cards are used extensively up here but I've never seen the attraction. Assuming one is fiscally disciplined enough to responsibly use credit cards, why would anyone want one?

We use debit cards for withrawals from the bankomat as well as to pay. Credit cards are not very popular. To withdraw cash from the bankomat with the creditcard means you are on the egde, because this is to expansive for common use.
The debitcard ist commonly on a cheap anual fee or included with the bank account, while a credit card is on extra fee and the business holders avoid them. You may use them as a credit card anyway - only banks interest is different. Slightly. :)