It's actually a feature to help preserve the life of the blades. The idea is that more rain will hit the windshield when the car is in motion and less when the car is at a stop. Therefore, if the wipers were to remain going at the same rate when the car is at a stop, there is a higher possibility that your wipers would be wiping a dry (or atleast a less wet) windshield. This causes the squeaky noise and eventually cuts down on the life of the blades.