Adults want children to read "serious literature" as part of their education. Too often, though, that means literature that preaches an unambiguous moral. For example, one of Shakespeare's weakest plays (both thematically and structurally) is also the most commonly taught play in our schools: Romeo and Juliet. Next up the line, both in weakness and in popularity, is Julius Caesar. Similarly, the schools push good, but black-and-white, books like To Kill a Mockingbird (puns intended) on kids. If one looks at the reading habits of kids who read for pleasure, though, a starkly different picture emerges. Harry Potter and His Dark Materials get read more than once; most of the other "popular" stuff gets read once, if at all. However, because they're "fantasies," they're labelled "escapist"... and not treated as serious literature by those not thoroughly trained in literature. (What that says about Don Quixote either the Cervantes text or the Menard text, take your pick is for another time.)
Conversely, when adults-who-read-for-pleasure buy books, they overwhelmingly buy books with distinct choices and no regrets (or, at least, none from any but the rare antihero). The Grishams, Clancys, Steeles, Browns, et al. wouldn't know a moral ambiguity in which all choices have adverse consequences if it slid up their collective leg and bit them on the ass. That is, adults are reading for pleasure precisely the kind of books that they rejected as children.
Why this mirror image? One possible explanation and I make no claims that this is definitive is that each class of readers-for-pleasure is seeking to escape from its daily world. The regimented world of children, with schoolwork that always has a correct answer, unyielding schedules, and a rigid place in the family, escapes to ambiguity. Conversely, the confusing, ambiguous world of adults, with choices that almost never have a correct answer, terrifying uncertainty as to what will happen tomorrow, and a constant struggle to establish and keep a social order, escapes to monochromatic certainty.
I'm not at all sure what this implies for the "value" of literature. It sure as heck has some important implications for both the law and the practice of publishing, though.