CVS

If I pay for something with my DEBIT card, I should NOT have to wait for my money. With a debit card, CVS receives their money immediately and there is no possibility that they will not. How is this fair as it takes DAYS for my money to appear? As long as I have my receipt, the receipt shows that there was NO problem with the transaction and CVS would immediately receive their money. Why is it a legal policy to put it back on the account the money was drawn from.