Because Islamic radicals aren't armed with American weapons. Because they have their own fight with Islamic radicals.
What does that have to do with the Arming the Saudi military? Nuthin'. No, we armed the Mujahaddin and bin Ladin was among them and they were on our side. They were fighting our enemy and using our weapons as we wanted them used. Not one Stinger missile has ever been used against a western target. We have not armed any terrorists.