It's my understanding that the jap mum was ordered defaced as per the ceasefire agreement between Japan and Allied forces (something the japs insisted upon.) Many U.S. commanding officers failed to enforce the mandate, thus many battlefield pickups returned stateside with the mum intact. Surplused Jap weapons sold commercially after the war usually had the mums defaced.
As far as I know there was no such terms written into the German surrender agreement. Nations such as Isreal and the USSR defaced the Nazi markings for their own reasons.