I understand what your saying, and it wasn't a big deal; I just found it odd. Now that you mention it, when I first saw the show I thought it was weird that firebenders didn't need a fire source to bend. But I came to think that firebending kind of balanced against airbending. In a world where every bender needs their element nearby, airbenders have an edge because their element is everywhere and easily accessible. Firebenders offer a balance in this regard. Earth and Water, while limited in accessibility, have an advantage when it comes to substance (mass). That's how it worked out in my head, anyway. The change did work well in the movie (as much as anything can work well in a movie like this.) It just surprised me.