Well, apparently the British court feels otherwise. It seems clear from this decision that these items are the property of the British Museum and they are not permitted to return them.
There are lots of ways you can lose property. Adverse Possession comes to mind, for one. The British Museum made no secret of the fact that they possessed these drawings and maintained them openly for a period of sixty years. Perhaps that is the justification for this decision by the British Court. I don't know, but I don't have to know. The thing speaks for itself.
I am sorry if you found my citation of Abe Simpson et al offensive, TheOtherOne. I was just using a fact pattern than may have been familiar to some of us here to show how this case differs from looting. This is a point on which some of us appear to be confused.
As for the moral dimension, I just don't get it. It was wrong for the NAZIs to steal these drawings, but it was absolutely not wrong for the British Museum to buy them. I guess if they had outbid the family, which was trying to recover family property, that would be one thing. But there is no indication that this is the case. They paid a third (...or fifth, or twenty-seventh...) party dealer for the drawings and conserved them. I don't know how one can fault the British Museum for that. If they had wound up in the hands of a private collector, would we even be having this conversation? The private collector would have told the parties to get lost, and rightly so.