Actually, M855 was never classified as armor piercing, since it doesn't meet the legal definition of AP. The US military does not, and never has considered M855 AP - which is why it's marked with a green tip ("other") instead of a black tip. The green tip is to prevent its issue/use with M16A1 1/12 twist barrels except in combat emergencies.
Why ATF specifically exempted it from the AP list is beyond me, unless it was to avoid confusion (not a step they take very often).