It's to track the sale. If you use a credit card Apple will store that info, and cross check to see if you've bought multiple iPads, which is against their policy. For some reason they feel morally obligated to prevent people from buying large numbers of iPads and selling them in countries where iPads are not available at a profit. In my country, it is ILLEGAL for businesses to not accept cash. They CAN reject credit cards and other fraud-prone pay vehicles.