It's pretty simple. You have a company, like the credit card company, which screens potential customers in advance and decides to accept a certain risk of double-spending or insufficient funds up to a limit. In exchange, the company receives a small fee from the merchant and reserves the right to bill the customer if there's insufficient funds. Merchants get instant confirmation when user swipes card or provides authentication they have an account. Just like a credit card company, the merchant gets paid whether or not the customer can foot the bill, whether it's 30 days later for a credit card or 10 minutes-a few hours for a bitcoin transaction.