That's because it's European and their laws are different than ours... almost 180° out of phase with what US law determines to be a "gun".
What classifies it as a firearm is that FN guns have the magwell on the upper and there is no lower reciever, it's a trigger group housing and reciever. FN guns have the action and magwell all in one reciever.
AR's have the magwell on the lower, the trigger group just so happens to be in the same chunk of metal.
Look through your safe, whatever holds the mag is serielized.
Old Sigs like the West German ones have the barrel, frame and slide serilelized to satisfy all customers, due to different parts of the gun were classed as the firearm in different countries.
But for the USA, the upper on a bolt action AR makes no sence to class it as the firearm.
The lower is the "gun" as it still holds the mag. The upper just holds the action.