It's not even incurring debt. If you run it as "credit", it still debits your account for the amount of the transaction. I've been in banking for 3 years and haven't found a single difference between "credit" or "debit" when using a debit card, other than it usually takes 1-2 days long to clear from a pending status when you run it as "credit". The only thing I can fathom is that when you run it as credit, the credit card issuer gets some kind of demographic information they can use.