The same way that SO many challenged the Davinci Code. A work of pure fiction, but everyone started attacking it as though it was an actual fact. I haven't read that yet, but people who have told me about it, do think it is true. How weird is that?
Well, here's the thing: Dan Brown has said publicly that he believes the Merovingian thing and wrote TDC specifically with the purpose of popularizing it. He's essentially just taking the thesis of a couple of books from the 90s,
Holy Blood, Holy Grail and
The Messianic Legacy and putting it in a fictional format that far more people will read--and even more will see, when the movie comes out.
Since so many people are reading TDC and accepting what it says as true (France is being flooded with tourists going to see the sites described in the book and trying to find the codes for themselves right now), concerned scholars, primarily Christian, felt the need to counteract Brown's claims with reliable history.