Why pay more than you should for a short term loan?

Mr Lender: Why pay more than you should for a short term loan?
 
Published: 12/05/2015 and written by Patrick Smith

Surely as a customer borrowing money you should only pay interest on what you owe at the time, right? So as you pay part of your loan off, you owe less, and the interest payable comes down accordingly. Why should you have to pay interest based on the whole amount you originally borrowed, even though you don’t owe that now?