The card's ability to downclock is not controlled by the motherboard, it's controlled within the card's BIOS.
It's entirely possible the card has an incorrect setting within it's BIOS. It could be a matter of it's P-States being set so as anything even remotely graphics-related, like Windows Aero, won't allow it to downclock. It could also be set to only operate at it's maximum clocks and voltage, with no other P-States programmed at all.
Contact Biostar and tell them it refuses to downclock while there is no 3D load and see what they have to say. Whether or not Biostar will provide you with another copy of the BIOS so you can self-flash it or if you'll have to send it to them so they can do it is entirely at their discretion.