But they clearly use coal for their prison platform shipyard thing, and probably also for their ships... Hmm. I bet the difference is that they have so many ships, and the ships are so big, that they don't want to exhaust the firebender complement of their crews, whereas... air vehicles (there are zeppelin things coming up in later seasons, right?) are probably much less massive (and also coal would be dead weight and therefore undesirable, whereas ballast is useful at sea).

I will keep an eye on that issue, to see what canon does with it and how it can be rationalized. Thanks for the heads-up!