Books for young readers aren't alone in this, I'm aware. Encyclopedia Britannica has at the beginning of their article on the function of mythology, "The most obvious function of myths is the explanation of facts, whether natural or cultural." Bernard Doyle, writing for the Encyclopedia Mythica over at Pantheon.org, had this to say: "Broadly speaking myths and mythologies seek to rationalize and explain the universe and all that is in it."
As I believe Owen Barfield said (in paraphrase), "Is there anyone so out dated that they are still teaching myth as stories made up by some fellow to explain how the world works?" (kazaraxa and plura can correct me on that if I'm attributing that paraphrase to the wrong philosopher.) The problem with teaching myth as explanation is that this is exactly how it sounds: there's no meaning behind the myths. Some clever storyteller or priest obviously just made them up to give a rational explanation to his peers, who bought all of it wholesale. While myth may not be the best place to use Ockham's Razor, it's hard for me to buy into the "someone just made it up" theory, given the widespread belief myths were treated to inside of their given religious systems. (As we already noted, myth = religion, according to Mr. Dewey.)
What are the alternatives? Barfield has a few (that I'm afraid I can't cleverly sum up in the space of a short blog entry). But it's Tolkien who I always return to here. C. S. Lewis confronted Tolkien with the definition that "myths are lies, though lies breathed through silver." Tolkien objected to this immediately, saying (as paraphrased by Patrick W. Curles) "There are truths ... that are beyond us, transcendent truths, about beauty, truth, honor, etc. There are truths that man knows exist, but they cannot be seen - they are immaterial, but no less real, to us. It is only through the language of myth that we can speak of these truths."
There is a difference between meaning and explanation. Getting that across in a book for young readers is a challenge, so I will try in my review not to hold their understanding of myth, which I view as flawed, against the books that put them forward. I'd love, however, to see some books come out for young readers that actually deal with meaning and myth, that introduce them to the wonder without saying, "But of course, all of that's nonsense."