I agree it's a pain in the rear, but I tend to think it's more of an automated process than the seller trying to charge you for items you don't want. You have to remember that most shoppers aren't going to have carts that will ring up to $0.00 and so they are simply going with the odds that you will need to pay something so they want to know how you plan to pay. Most times though, the site also has an option for paying by phone and/or mail. If I know my items will be free, I'll usually check off that one