Correct, I believe the charge should be "Receiving Stolen Property".
I see your "at what point does it end" opinion but the gov't shouldn't be able draw the line just short of itself for the sake of "posterity".
It is not really receiving stolen property. They were not buying it from the thief. They were buying it at a garage sale, or some such thing. If the British Museum had not bought them, they would have wound up as a placemats in some Bavarian diner or something.
Europe was in ruins. It was a good thing that somebody picked up some of the pieces. The British Museum should not be faulted for this.