If Dr. Grace Augustine, is supposed to be the most renowned expert in the life and ways of the Na'vis, (she wrote "the book" remember) then why would they come out of the incubation hut wearing belly shirts and street clothes?
It must have cost the earthlings trillions of dollars to develop the DNA sequencing and all the neuro science required to create the avatars in the first place... didn't Dr. Augustine have enough budget and attention to detail in her studies to create authentic-looking Na'vi loin cloths?
If you really wanted to go through all the trouble of BEING a Na'vi (tail and all), wouldn't you also want to go out wearing the thong and try to dress like one too?