Well its because they used a small clause in the law & you can thank the polarity of the AR pistol for it . You can thank Olympic Arms for starting this , they were warned making a pistol of the AR ,that something like this would happen & that was a long time ago ( IMO has no strategic value at all, except a FUN gun ).
What I can't understand is ,if it can be used in a pistol , why not ban it from pistols made in that caliber , why the over reaching & banning it all , since there's no law against AP ammo in a rifle ? <dontknow>
They have over stepped their authority , once again !