The reason for taking any post like that down is simple, it is illegal to falsely accuse people of lying, including on the internet, that is defamation of character. And generally if an accusation like that is made with no evidence, it is considered false. In this case of course there is no evidence because we are not involved in any conspiracy, and besides that, nobody knows enough for certain about XMRV to lie about it, we are all speculating based on a little data...
Defamation is serious trouble, any forum and the individuals involved can be sued for that type of posting.