Its not illegal-- The ATF does not make laws.
The parts that are illegal to possess are <most, if not all> M16 parts and LE-Only stamped parts, such as the lower receiver and hi-cap magazines. If the other parts were illegal to own, while they are stamping those parts, they would have stampped those with "LE-Only" as well. Also, think about this: you can still buy pre-ban parts (including complete uppers) from various manufacturers like Bushmaster, DPMS, Olympic Arms, etc. Why would Colt be any different?-- they aren't.