col.lemat,
All is explained in that link to Tapco's web site page in post 2.
Basically, it is a law that says you can't build a semi-auto rifle or shotgun here that you wouldn't be allowed to import from a foreign country because it is a military style (non-sporting) weapon, UNLESS that firearm (or its parts, rather) is made in the USA. "Made in the USA" in this case is defined by how many out of a list of constituent parts are made overseas. If more than 10 of the primary parts were made overseas, that's a foreign-made weapon and it wouldn't be importable.
So the companies that make and sell things like AK-47s, AKMs, AK-74s, Galils, and various other military style semi-autos have to add a number of US-made parts to get the "foreign parts count" down below 10, so we can pretend that it is a Made-in-the-USA firearm.
Usually that means the AK you buy from your local gun shop will have a US-made receiver and trigger group (it needs those anyway to become a legal semi-auto), and then maybe a US-made pistol grip or gas piston too.
For us home-build types, we end up buying, for example, an AK parts kit and US-made receiver to build it up on. Then we have to replace a couple of utterly irrelevant, pointless things with US-made parts in order to be legal.
It's stupid, but that's how it works.