Again, there is an important factual distinction in the present case. Heroin, crack, stolen art, etc., are all contraband items, the possession of which is illegal in and of itself. These are the items upon which the existing case law is based. The only market in which these goods are traded is the black market, so it makes sense to assess them with their black market value.
Here, we have an item that is legal to possess, and was obtained legally in the normal manner in which artwork is procured. While its possession and sell has been subsequently banned by federal law, the federal government has given them a waiver to possess the item legally, while still, ostensibly barring its sale.
So the question facing the court is "for estate tax purposes, should an item that is legal to possess but illegal to sell be assessed its FMV based upon its black market value or its value on the legitimate art market?"