The issue is that under the current interpertation of the 'rules', as outlined in the Code of Federal Regulations (which is the guide for how Federal laws are implemented) , importing parts kits with barrels is NOT allowed, unless the barrel is to be used for a replacement on an already existing firearm.
In order to comply with these regulations, importers MUST indicate on the Form 6 (firearm / parts import form) , that the part kits with barrels are being imported to be used as replacement parts for already existing firearms.
If the importer is importing the parts kits to be used to manufacture new firearms then they CANNOT import them , and if they do import them with the purpose of building new firearms they face criminal prosecution for falsifying the Form 6.
As long as the importer sells the parts kits (with the barrels) as REPLACEMENT PARTS ONLY, what they are doing is 100% legal.
Now if you purchase a parts kit and use the kit to build a firearm YOU have done nothing illegal.
When they place 'FOR REPLACEMENT PARTS ONLY' in their ads they are just making it precisely clear the purpose for which they imported the kits (with barrels).
It would also be LEGAL for an importer to bring in parts kits without the barrel,to use the parts to build new firearms with.
So it is legal to build your rifle from the kit, keeping in mind that you still have to comply with the AWB - until Sept 14th - as well as the Nov '90 import ban that requires five or six US made parts in an AKM type rifle.