I have a debit card. I always kind of wondered why I was asked. I take my debit card out when I want to use debit and I take my credit card out when I want to use credit. (Yes, I know that a debit card can be used as either, but in my own head I am clear about my intent!)
Then I kind of figured out that it makes a difference to the vendor. Credit card fees apply to the seller, they don't just make money off the interest charged to the buyer who doesn't pay off the full amount each month. On the other hand, the credit union loses with debit use and makes the fee from sellers so they always encouraged members to use the card as a credit card.
this article about 8 dangerous places to use a debit card.
Who knew? But I will congratulate myself for having good instincts anyway.