The stock is a stock, whether or not it comes on an M14 or separate. The Tavor is a bullpup rifle; there is no stock to take off. It IS the rifle, and is not specifically prohibited, hence why it is allowed.
Now if the phone jockey at the firearms center says you can have it, expect:
a) it to be kept at customs and never given to you, as it is a prohibited device that you are not licensed to own, and possibly face criminal charges for trying to illegally import it, or
b) the cops will come take it from you without compensation after they realize they made a mistake.