Every OS is RIM-based. RIM gives the base device software to the carrier devs and then they adjust it accordingly (for example, see Verizon's GPS API deal).
Since every device software is tailored to each carrier and its intended device, how would anything but the device software driectly from AT&T be official?
Straight RIM device software is still considered beta and thus not official. Its RIM only/internal for a reason.