I've noticed that. But here's something you didn't notice. Unlike the book which is classified as fiction but claims to reveal "truths", these websites state things which can be found to be true.
These websites actually have a backing - that is, they have historical sources. Whereas the Da Vinci Code is a fiction weaved with half-truths. It's these half-truths which are misleading.
Read the websites, and reply.
Have you read Anne Rice's Vampire Chronicles. If you have, you'd find they also claim to speak the truth (that is, the vampires). And there are some things which are true in the texts as well. Does that mean the book is to be taken as an encyclopedia, like some have taken to in reading the Da Vinci Code?
These days people read, and don't think (enough). They want instant gratification (movies, tele are a prime example).