That baffled me for a while till I found out about factoring and the sale of receivables. Companies that provide you the big screen tv actually sell the account that you formed and the agreements that support that account to a finance company. That finance company essentially lays in wait for you to come to the end of your “no payment holiday” and then hopes you will not be able to pay out on time. They then start the interest and principle calculations from the day of purchase and you pay it all right then and there. If you cannot you start paying the debt off over a period of time at a substancial interest rate.
The company that sold you the item gets paid within days of selling you the goods.