Kokua Line: Is debit-card service fee legal?

Question: Even if I pay in person at a satellite city hall or the DMV, they charge a fee to use a credit card or even a debit card — it’s not just online or at the DMV Now kiosk. I asked why a fee is added to the debit card since it withdraws money from your checking account; nobody had a clear answer. Maybe because it’s backed up by a credit card company? That’s dumb: The check can bounce, the debit card will decline on the spot.