Some people rob banks, but that doesn't mean robbing banks is legal. And some vets brought back U.S. weapons, but that doesn't mean it was legal to do so. It was theft of government property, period. Keeping allied weapons was also illegal, but not many Americans acquired those since U.S. contact with allied forces was minimal.
I can tell you from personal experience that while those vets of WWII were heros and "the greatest generation" a lot of them were also the greatest liars. I wish I had a dollar for every vet I knew who got a pistol from a captured German Field Marshal, or who took a sword from a dead Japanese general. (The Axis powers lost because they had all generals and field marshals, no privates!)
I learned to "read a chest" at an early age; it helped when a "Marksman" badge (the lowest rating) was being passed off as a Distinguished Service Cross. Soldiers who never left the states claimed to be combat veterans; one told me he had captured Hitler but the SOB got away!
And of course families helped perpetuate stories about weapons, as Dfariswheel says. One officer's widow showed me the carbine her late husband "carried on D-Day." It was a Model 1873 carbine.