The fees on CREDIT cards are confusing, with the percentage changing based on volume, etc. I once found a really good explanation online, and it made perfect sense for about 15 minutes, and then it faded again.

Debit cards usually have a fixed fee per transaction, as well as a percentage based on volume. Credit cards don't have that fixed fee, just a percentage of the total, so they don't care if you use it on a small purchase.

But if the fixed fee is 50 cents, and you buy something that is 50 cents, they literally lost money. Even in a case where the item is 2 or 3 dollars, that 50 cent charge is a big hit to the profit. Do that 100 times a day on things like cans of soda and candy bars, and much of your profit on those items gets eaten by the fees, and those things are a BIG part of the profit for something like a convenience store.