It's due to as you say the english/british background of the books. I don't see any black elves, or such. That they want light skin does seem abit odd, when the hobbits are farmers, so should have more sun tanned skin. The elves should be extremely light skinned as they are not out in the sun as much. The casting agent isn't being racist, he isn't saying he only wants WHITE people, he is stating light skin, because there is an asthetic that is trying to be created for the movie. Just as they have a height restriction for the hobbit roles, it's another restriction. Such is the movie industry. For background workers where your basically part of the set, the decoration your judged on your asthetics for the scene, if your a main actor your still judged on your apperance but also your acting skills.