Not All Copies Are Worthless
It is no surprise that a copy of the Mona Lisa - albeit, one that was painted at nearly the same time and perhaps in the same studio as was the original - is worth something. Even considering that the copy is purposefully different - the lady with the mysterious smile appears younger and perhaps more at ease - value accrues to an object that has an interesting story, regardless of the authenticity of some other facet of that object.
To mentally test this hypothesis, consider the knock-off Louis Vuitton hand bag famously used in the movie Hangover 2. That use resulted in multiple lawsuits and damages claims from Lous Vuitton and the story was reported world-wide. A collector of the controversial and morbid would certainly pay good money for that item.
The key here is that that collector would be buying an item whose owner proclaimed its inauthenticty rather than misrepresented it, and that purchase would not carve a penny out of Louis Vuitton's market share.
Food for thought.
Anyways, read the original story about the Mona Lisa here...