Yet it IS the law.
No there’s where you are wrong. It’s not the law, it’s the in house regulation, and apparently not at all ever offered for public hearing, as required by law for comment and revision, just “secretly decided” by some Bureaucrats the ATF&E. At some point in the design of the AR-15 the designers were told where to place their serial numbers by the ATF (no E) back then. . . And they were told wrong. The regulation does NOT meet the explicit definition as laid down in the statute for regulating firearms. Now perhaps the statute was poorly written, and I believe it was not only poorly written, but unconstitutionally written, but that is because it was written by idiots who do not at all use or understand guns, much less understand them constitutionally, but that is ALSO a question we need to address at the same time as we call these guys on the carpet about the whole mess.