The taint was cased by the summoning of the demons of the Seven Deadly Sins. The purpose of letting these demons loose on the world was to bring magic into the control of the cabelists. It never works that way.

The real effect was to taint the very magic field with evil. Those that practiced magic without a very firm grounding in ethics were drawn to the less savory arts and actions. It time it was believed that the practice of magic itself was evil. It certainly led down that road.

With the destruction of the demons and access to the magic of Greyhawke the taint is seen for what it is. A passing phenomenon. Dangerous, make no mistake, but is is something that can be cleansed.