According to the department of commerce, there is a difference between manufacturing and assembly of which both have distinct definition in the law. He perhaps should have broken down his services and sales better on his invoicing and he could have avoided any problems. He could have sold the 80% lowers and a separate kit of upper receiver parts, or even a completed upper, along with a trigger assembly kit. He could then rent time on his Computer assisted Machinery to finish the 80% lower, and also charged for his expertise as an advisor to assist the customer in assembling their own AR. Thus, the customer would meet the letter of the law in making their own gun. By billing it out that way, Roh would not then be the manufacturer of the firearm, but merely a retailer of the parts, a renter of machine and bench/tool time, and for his expertise as a technical advisor and trainer on how one can assemble a firearm, all perfectly legal to do. Thus, no crime, no time.
The ATF had already ruled that pushing the start button on a CNC does not qualify as a person finishing his own 80% lower, and that was not disputed in the court case. What was disputed was the definition of a firearm frame or receiver.