Actually this makes sense to me. I mean, the other three kinds of benders don't generate their elements out of their own bodies (except for Katara bending her own sweat, which isn't quite the same thing); as a rule, they manipulate the pre-existing substances in their environment. In the show, the Firebenders are the exception to that rule. That's somewhat justified by the identification of chi with energy and energy with fire, but the movie's approach here just seems more self-consistent.
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.