What I don't get is where's the sense in having a law that requires you to swap all this stuff around based on where parts were made?
Since you are from England let me try to explain.
Because at the time of the law that branch of the US government had greater control over regulating imports than domesticly produced items.
The president could not simply declare an "assault weapon" or semi-auto ban (self loading in the UK, not full autos or selective fire) by himself. He could however impose any type of import restrictions.
They aquired that power through ITAR or International Trafic in Arms Regulations.
ITAR when passed was never about small arms. The entire focus of the legislation was on restricting classified material that gave the US a technological advantage.
A classified sensitive part on a modern jet fighter, or the latest missle guidance system for example.
Most of those types of things are primarily designed and produced by private individuals and private companies in the United States. There was nothing limiting them from say creating profitable contracts to work with another nation and develop modern defensive technologies or export existing ones.
So ITAR was passed to allow the government to restrict classified material produced by the private sector, like many privately owned government contracting businesses.
Once passed however officials began to exploit it in ways completely out of the context of the original legislation. Restricting small arms utilizing technology already known to the entire world for example. It was never about guns, but because of the name of the legislation politicians were able to expand gun control through ITAR.
Now ITAR has expanded to have a significant role in the firearms industry.
A domestic ban or restriction may take an act of congress (and congress members who ignore the oath they swore to take office) but restrictions on imports or exports do not. Those can be changed simply through a policy change or an executive order.
Then comes the hard part of defining what is or is not a firearm subject to those easier to pass imported (or exported) firearms legislation or policies.
That parts list is the result. It defines what a foriegn firearm subject to those laws is.
A foriegn firearm is a firearm with more than 10 of the listed parts.
So while it remains under the definition of a foriegn firearm there is stricter restrictions on it because it must still comply with the import restrictions. Once it no longer meets the definition of being a foriegn firearm it is no longer subject to those easier to pass policies and restrictions, and only subject to federal and state law on domestic firearms.
Confused? You should be, many people have no idea when they buy such things from the store. They can legaly buy any magazine or normaly unrestricted or uncontrolled component but combinging it with thier gun is a major crime. Yet thier friend who purchased an identical firearm made in the USA can do the exact same thing without breaking the law. The average lay person is thus randomly and unknowingly commiting a major federal felony or not simply based on the origin of the rifle.
