That's a good point. Maybe it will be changed in 3.5. It seems they are looking to take some spells away from transmutation in order to better balance the schools for the new specialization rules (e.g., teleporting spells are now conjuration).
you can justify any effect under transmutation. Just say it is changing air into actual fire, or changing air into a substance that combusts when exposed to air or whatever.
Its effect fits evocation best of course but the special efect of how the flame gets there can easily justify it being trans.