The Obama Administration's reasons...

...for barring the return of the Korean Garands, according to the South Koreans, are...

“The U.S. insisted that imports of the aging rifles could cause problems such as firearm accidents. It was also worried the weapons could be smuggled to terrorists, gangs or other people with bad intentions.”

The short answer to that is bullshit. Here's David Kopel's longer and more useful response.