Somebody proposed it and the powers that be (National rallyx board) liked it enough to approve it.
Stock class rules about scoops and wings have not changed from 2011. The only thing that might allow the addition of a non US OE spoiler in stock would be section 5 of the stock rules.
6.2.C.5. Dress-up and convenience items which do not give a performance advantage, reduce the weight of the car, or weigh less than the replaced standard part they are replacing are allowed (i.e. shift knobs, pedal covers).
I am not sure if spoilers fall into that group but it is the only rule I see that might make them legal.
It is just MA for 2012 to align all the All wheel drive classes with the same letter since they now have F and R for modified also instead of just M2.