Satan, People and Sin

From John Piper's book "Life as a Vapor:
O Lord, Your ways are not our ways, and Your thoughts are not our thoughts.  Yours are unsearchable and often baffling to our finite minds.  You have chosen to give Satan freedom to do great damage, when it would be no injustice to him to destroy him now.  We bow before Your wisdom.  We embrace with Jesus the cross that saves our souls and sends us into battle with faith in the superior beauty and worth of Christ.  Go withus.  Help us.

Why did God permit the Fall?  How could sin spread over the world that God made good?  Why did God just put a "Do Not Enter" sign over an open door to keep Satan from disrupting His world?  Why not just close the door?

What is your answer?