"Skirt the rules?!"
It's perfectly legal for me to make any weapon I want as part of my right to bear. They can only regulate the sale, which is why there's a distinction of when it becomes a completed gun so that they can regulate it. If I'm making it for myself there is no reason to give it a serial number or register it in any database anywhere. All this part does is provide a shortcut, much like the off-the-shelf engines and tires the Orange County Chopper guys use in their custom bikes. It doesn't mean I didn't make it myself, it just means that I didn't make it from scratch. Requiring me to make it from scratch would be impossible. How of the OCC guys ever going to get DOT-rated, safety-tested, expertly a engineered tires if they have to make it themselves? And where does it end? Will they have to mine and refine their own steel/aluminum? Build a CNC machine from scratch? Anything up to just prior to final assembly is a nebulous distinction. If only gun manufacturers were allowed to make guns then our rights are at their mercy.
Disclaimer: I do not nor have I ever owned or made a gun.
Hypothetically, if you bought that EP you didn't make a thing. You removed the white part from the black part.
You are correct that you are allowed to manufacture any gun you want without any sort of license for your own personal use. No one is disputing that, not even BATFE.
To borrow a food analogy, they're saying that you aren't a butcher just because you pulled the bone out a spare rib you bought at Kroger.
No one is requiring you to make anything from scratch. The BATFE publishes guidelines for what constitutes an 80% receiver for many types of firearms and if you have a specific one you want to inquire about you can always send to the Tech Bureau for a letter.
You need to go back and read my original post on this issue and see what really constitutes an 80% AR receiver. Then you will really understand why the BATFE was right here.
ETA: for clarity's sake I meant you can manufacture any gun that is compliant with NFA1934. You can't manufacture a fully automatic gun, nor a SBR or SBS for personal use without the appropriate stamps. And JSYK BATFE won't issue stamps for fully auto weapons except for ones registered prior to 1986.