Because in the case of people like them, they might use you looking at the shiny truth to do things that aren't so shining. Not every lie is like "Santa Claus" at Christmas. If it looks beautiful and fun, then that just means it's that much easier for them to do horrible things behind the scenes. If it really is the truth, scrutinizing it will make it obvious it is. Lies don't hold up, but the truth does.