Here is the logic so far as I understand it:
M14 and rifles like it that were designed to be fired conventionally were meant to be fired conventionally. Whether they fire with stock or not, they were designed to be fired that way and stocks/chassis/whatchamacall'ems that change this operation to bullpup are illegal. Why? The logic is that they make guns shorter, therefore concealable, therefore more baby killing and probably the adding of bits that may alter fire control that may give you full auto or something like that. Stupid? Yes, but you better be good at law and pushing indifferent people if you want to change laws.
What about the RFB? The RFB was built from the beginning to be a bullpup and turning it conventional would require more than just a slap on stock/chassis/thingymajigs and because of this, it is true bullpup.
Honestly, I don't think they had the M14 or any .308 rifle in mind when they penned this law. I think it was meant for drop in stocks for the mini-14 and other .223 rifles like the AUG.