What makes a loss prevented a loss prevented?

This is a screenshot of the game I just had. ( http://gyazo.com/b1aedd2c25ea6d2cbad88552f8ae488e ) Our Soraka support quit at 15 minutes and our top Riven intentionally fed the whole game. I think it is quite unfair that in my series I get 2 players like this and it counts as a loss.. I don't deserve a loss because my team quit on me. So I ask, what really makes a loss prevented a loss prevented?