You can think of Pending Amazon orders as a transaction period that benefits and protects both the buyer and the supplier.
Every time a customer places an order it assumes pending status for at least thirty minutes before it is approved. It is basically intended to give the buyer time to think through his order once again and cancel it without any hassle if he so wished.
At the same time, it gives Amazon time to verify the customer's credit card so as to reduce the chances of fake orders. So, in a way, it is Amazon's idea of providing a safe environment to its buyers and sellers.