Product is product. In creating a transaction, creates a debt. If I am using a credit card to pay for the transaction, they cannot refuse the sale, it does not matter if it is a condition of sale.
Technically you aren't in-debt to Apple until you buy the product. So, you can never get to the "in-debt" stage (generally, since some stores are apparently accepting cash) when buying with cash because they won't allow a transaction to begin with. With the debit card, you're taking the money out of your account and putting it in theirs - so no debt needed. With the credit card, your debt is with the credit card company, not the store.