Cant download my updates or download anything free

I cant download any of my updates, or download anything free if the bank card that is connected to my apple account gets down to a zero balance, WHY. I went to my local cell store and they said it should not matter what my balance is on my card if it's free or a download it should go through. A friend of mine at work says her bank card has went negative before and it did not affect her downloading ability. What do I do???