In Chapter IV ("The Act of Expression") of Art as Experience, John Dewey writes:
If one examines into the reason why certain works of art offend us, one is likely to find that the cause is that there is no personally felt emotion guiding the selecting and assembling of the materials presented. We derive the impression that the artist, say the author of a novel, is trying to regulate by conscious intent the nature of the emotion aroused. We are irritated by a feeling that he is manipulating materials to secure the effect decided upon in advance. The facets of the work, the variety so indispensable to it, are held together by some external force. The movement of the parts and the conclusion disclose no logical necessity. The author, not the subject matter, is the arbiter.
By "emotion" Dewey perhaps means something like "intuition" or "inspiration," the specifically artistic emotion that determines "the selecting and assembling of the materials." In experiencing a work of fiction that seems "regulated by conscious intent," the reader does not share or him/herself intuit the "emotion" that holds the work together, but in a sense only observes the author manipulating the "materials"--the story, the depicted characters, etc--to achieve an effect "decided upon in advance." In this kind of inauthentic, aesthetically impoverished fiction, the writer has latched on to fiction as a vehicle for "saying something," not as a form of verbal art in which the work must "say" for itself. Readers expecting literature to provide a compelling reading experience unlike that to be found in ordinary written discourse are rightly offended by this kind of fiction, which privileges the author's intent over the achieved "necessity" of the work itself.
As Dewey writes further:
In reading a novel, even one written by an expert craftsman, one may get a feeling early in the story that hero or heroine is doomed, doomed not by anything inherent in situations and character but by the intent of the author who makes the character a puppet to set forth his own cherished idea.
Suffice it to say that characters can become "puppets" in any kind of narrative, not just a tragic one, although Dewey implies that the tragic mode is especially susceptible to didactic purposes, resulting in a work in which "doom" is not "inherent in the movement of the subject matter portrayed" but instead creates "an arbitrary and imposed world." Tragedy becomes not the result of human frailty or conflicting goods but a consequence of the machinations of wicked people or oppressive institutions or political malfeasance. It is a particularly tempting narrative convention within which to cloak one's "cherished idea," although obviously such ideas could just as easily motivate (and perhaps be even less concealed in) satire, especially satire of a more overtly corrective kind. And both satire and "tragedy" as Dewey describes it here easily enough slip into moralism:
It is for similar reasons [those reasons why we "resent" being presented with characters-as-puppets] that we are repelled by the intrusion of a moral design in literature while we esthetically accept any amount of moral content if it is held together by a sincere emotion that controls the material.
The moralist is the writer who has a "moral design" on literature--who sees it as an forum for moral discourse more than as an aesthetic form--or who wishes to create a moral "design" in works of fiction or poetry in the guise of, or in substitution for, aesthetic design. We are "repelled" by the highjacking of literature for these moral purposes because it is not "sincere." It has no regard for the integrity of works of art as art, reducing both it and real moral discourse to a kind of cheap thrill.
I think Dewey's distinction between "moral design" and "moral content" is very important in the consideration of art and literature. Focusing on moral design essentially converts works of art, especially literature, into a method of conducting moral/political debate by other means. At its worst, it becomes tedious moralizing. To speak of the "moral content" of literature, however, is to recognize the inherent moral quandries and conundrums that fiction and poetry inevitably explore, simply because novels and stories and poems are written by human beings and are inescapably about human reality. Some readers might indeed want to abstract "moral content" from particular works to ponder more fully, as long as the work in question "is held together by a sincere emotion that controls the material." That is, as long as it is, first of all, art.