Only way i could see tieflings retaining their fire abilities is if the source of their magic was the actual demon (who i would consider to have demigod status). The curse not only drains races of their abilities but also drains spells as their being cast. Effectively you have to put in 3-4 times the amount of magic for most spells.

Now if you're saying tiefling are working off borrowed magic from the demon, you could say the demon has more than enough power to spare, thus can easily spare enough magic to overload the 'simple' spells tieflings are using.