I was of the impression that the tangs of many family swords were cut off to be standard military length per Japanese military regulations. I could definitely be wrong about that and would certainly invite correction of that notion if wrong, as would, I'm sure, the WWII collector from whom I heard it.
I would expect that the tang was cut off by a Japanese arsenal to make the sword fit Japanese Army standards. I doubt seriously if a GI would bother or even have a reason to do so. So I think the GI with a hacksaw is off the hook here. The number one vandal of fine (even exquisite) Japanese swords of all times was the Japanese military government during WWII! Thousands upon thousands of heirloom swords ruined (as far as collector value).some more WWII posed pics, with a Yasukuni Shrine forged Gendaito sword, would've been one of my most prized possesion if some GI hasn't sawed part of the tang off
Then again, he probably went through WWII and who am I to judge him?