Do you have a source for the rights being sold? I've only been able to find evidence that it was licensed, and the status quo for copyright is most photos have one and don't transfer it automatically on sale of any particular copy.

Unless there's an ulterior motive like trying to set a precedent to solidify the position of other works on shakier legal foundations, I don't know why the Warhol Foundation wouldn't take such a clear-cut defense if they actually owned those rights.