Why do children's books have religious imagery interspersed randomly?

Recently, I have been taking my 3 year old son to the library every weekend to pick out books. Typically, I don’t screen them since I want him to feel the freedom of picking his own books. In any event, these are children’s books, not adult books. What I have come to discover is that many books have either Christian themes, outright prayer or imagery randomly placed within the book. To me, this is outlandish. Why should a story about a dump truck have to refer to Christmas or a story about a girl who flies have crucifixes in the illustrations? I don’t want to have to screen every page of every book (he picks 10 at a time), but perhaps I have to?

Edit: I’m not talking about obvious ones, like “How the Grinch Stole Christmas”, but just random, out of context religiousity.