Going to say this before I start, I am fully aware of the 1-5 levels of you being a human, than again, thats a flashback, so you're char doesn't start off as a human, just shows you a flashback of back when he/she was a human.
Ok now I will explain why they are not human. The darkflight racial says the worgen form is their TRUE form. Also, when they die and in spirit form, they are worgen. Also, as a quest in gilneas says, any form of excitement/anger/pain etc. will revert them back to worgen. And when fighting, they turn into worgen. The human form is either one of two things : a magic effect that they have to take a potion each time to take on or it is an illusion, like dragons in wow when they turn into humans, that they have to have complete focus on in order to retain it. Also in this video
http://www.youtube.com/watch?v=OZBS0PPgFTM at 23-30 blizzard says "the worgen, who were formerly human". Formerly, meaning they were human, but are now no longer human, and are now worgen. Also, why would blizzard give the alliance another bunch of humans? that would be stupid. They aren't werewolves either, werewolves are only wolves at night, worgen playable are worgen truely, and at any time of the day or night. Also, arugal didnt create the curse, the pirewood village and wolfcut worgen are wereworgen created by a mutated version of the curse by arugal. Here is what actually happened to the gilneans
http://www.vcsale.com/Worgen-and-Gob...lysm_1821.aspx read the worgen flavor text there. It says the people got turned into worgen from being near the actual normal worgen, and the curse spread just like that, being near a worgen causes you to turn into one. Anywho, thats all, just wanted to clear up that the playable worgen are worgen not human anymore.