Where do you park your car? I'll gladly steal it and return it to you for a finder's fee (or buy it "hot" and do the same).
I see the point you are trying to make. However, unlike this mask, my car has a VIN number, registration history, and a bill of sale in my possession.
This museum in Missouri possibly saved this mask by buying it. It might have ended up buried in some private collection lost to the public, and Cairo, forever. For that should they be punished to the tune a ½ million?
Granted, I don’t know how easy or hard it would have been to review the history of this mask before buying it but I hardly think that if the Museum of Cairo is so disorganized that it can’t keep its property from disappearing that it should be able to rely on the budgets of the worlds museums to recover these pieces without compensating them.