""Tawfeeq, who occasionally visits the United States, was surprised to learn shoe throwing was not considered an offense in North America. A search of literature reveals references suggesting the power of the shoe is ancient. For example, in the King James Version of the Old Testament, Psalm 60:8 says: "Moab is my washpot; over Edom will I cast out my shoe: Philistia, triumph thou because of me." "Perhaps it has to do with the hierarchy of the body position, that is, the relationship between the head and the feet, the head being at the top and not touching the ground and the dirt," Shirazi said. "The head carries a more prestigious status in comparison to the feet, which in older times mostly remained bare.""" (thanks Yazan)
PS I can confirm Tawfeeq's discovery. I have lived in the US since 1983 and have been throwing shoes at neighbors and friends here and not one American has ever taken offense.