Miss Manners: Is it rude to allow a line cutter without asking the line?

Dear Miss Manners: At times, a grocery checkout line will be very long, with only one cashier available. Someone with a full cart, three or four customers ahead of me, will offer a customer approaching the long line with only a couple of items permission to go ahead of them. They thereby award the same permission on behalf of all customers behind them in line, even though we were not asked if we would mind waiting a minute or two longer for the single-item customer to check out.