I was thinking about this after the last patch came out, and begin to wonder if it was intended since it wasnt fixed and seems like one of the easier to do's on the to do list.
But then I got to thinking about how the Sedna High Priest bug is still in, and theres no way its intended for a 3rd tier cheaper unit to get more bonus from a passive skill, than the high end more expensive unit. So theres plenty of things that they just haven't got to yet, and fire mode may be one of them.
However, I do have a theory on why it might not be a bug, which others have already touched upon. We all know about the great boosts we get from form switching, and we all know the drawbacks on chases and how frustrating it is to not be able to get off those one or two critical hits in fire mode.
It could be that the Damage buff from Fire to Ice switch, and the Attack speed/Damage buff from Fire Aura is too strong in the Dev's minds, as CosMoe said, and they didnt have enough time to think of a good solution so they just put in the lack of a chase auto attack for fire mode. I actually believe thats a strong possibility. But it could be true its intended and working right, or just an unfixed bug as well.
The real problem is that our only link to the minds of the Devs as far as answers has come from very few sources (ok, mainly FrogBoy--- and we love you for your input). But I do think its likely that its intended, whether a permenant intention or not. Because if you got the damage and attack boost from fire aura and the damage boost from Fire to Frost form switch, constantly, and able to give chase with it....that would be pretty nasty and not even relying on any items. At auto attack focused items, and you could become an insane Auto attack TB...and I seriously doubt they want him played that way. He is the mage afterall, and he has enough spike damage without a 3 skill point investment allowing him to auto attack with the best of em. Thus I believe its intentional. But like I stated before, I believe its there because they couldn't figure out how to balance it without having to completely rework some of the skills or the form change mechanics.
Being auto attacked by Frost TB, and hit with Rain of Ice can be a killer (well a good finisher with the dmg boost) as it is. Immagine the super fast Fire form TB able to auto attack you, for even MORE DAMAGE, more frequently, and get in fireballs every 5 or 6 seconds, still able to catch up with you because of his fast speed. Would be pretty brutal.