If I'm correst, I seem to remember that this rule dates back to the time when the US market was flooded with surplus military rifles of all descriptions, sold at junk prices. This included the infamous Mannlicher Carcano used in the Kennedy assassination.

The Gun Control Act of 1968 banned the import of all military weapons, and the collector exception was passed later, I believe in conjunction with the introduction of the Curios and Relics license issued to collectors by the BATF. Certainly the definition of Curios and Relics with regard to military weapons matches that of the unlatered military weapons which are allowed to be imported.

A little known fact is that holders of C&R licenses are allowed to import C&R eligible firearms themselves without having to run them through a regular dealer.