I believe this means the opposite of what you think it does. You aren't able to charge your cell phone bill to your iTunes account rather your iTunes purchases (if you do not have a remaining iTunes balance) can be charged to your cell phone bill. If the day were to ever come allowing your cell phone bill to be paid with iTunes credit you can be assured that the sweet iTunes gift card discounts that pop up for 70 to 80 cents on the dollar will disappear.
Thanks - I see now exactly what the OP means now thanks to your clarification.