Because inclusion on the NIH website, in the same general format as peer-reviewed articles are given, can give it the aura of being promoted or at least vetted by NIH, even with the disclaimer.

If you look through Snopes there are tons of examples of hoaxes and false gossip that end up spreading because they are found on an "official" website (or official-looking website), even if that official website is just a library where anyone can publish.