Don't go all Clinton, "depends on the meaning of what is, is", on us. If there is no nuke material it is not a nuke. Also, the explosives used to initiate, several essential control elements, etc are also regulated. Your rationale would be like saying you could have all the firearms you want without restriction but you can't have firing pins, barrels or ammunition.
That's why. You can "own" one, you just can't get the nuclear material.
But you already knew that.