It's because of import regulations, they need to contain no more than ten foreign parts (from a list of specific parts). For most models, this requires installing 6 US made parts, commonly the trigger group (3 parts), receiver, muzzle brake, gas piston or pistol grip. Some sellers use a US made mag which counts as three parts (body, follower, floorplate) but this can lead to problems because you are in violation if you swap it out for a surplus commie mag.
If you hear someone talking about 922(r) compliance parts, this is what they are referring to. You can get a lot more information about it at this link.
