In the comments section of a really good essay on Spike Lee's best and most widely debated movie is this gem. I have excerpted it here in full, because it deserves it, and because I'm about to go off on some major tangents with it.
I think the point on why audiences expect films to moralize is kind of simple: we use film, in America at least, to live vicariously through others so we don't have to engage in the actions ourselves. Most films acknowledge this, as there's a distinct narrative difference between false actualization and legitimate call-to-action. Most American cinema falls into the former, while something like DO THE RIGHT THING pursues the latter, which is also why that film troubled many American cultural critics (and audiences) in the time of its release.
As an example, America doesn't want revolution; it just wants the explicit promise of it, and often a constant stream of entertainment that feeds into that narrative. It's why THE HUNGER GAMES is so popular. We're pissed about what our country has become, but we're too lazy to do anything about it; so Americans can live through Katniss as she does the things most of us only fantasize about. This is why most post-apocalyptic fiction and revolution-leaning cinema are infantile in the way they handle the scenarios they propose, at least when compared to much of what Lee has done as a director. Lee's films often end in frustrating ways, such as with SCHOOL DAZE, where the central problem isn't resolved, and, in fact, transfers its righteous anger from its characters onto the audience and expects them to follow up on what the film was attempting to accomplish. You aren't allowed to feel like anything was finished or made better, because, realistically, nothing in life ever is — we spin in endless cycles of mindless violence, racial inequality and nationalistic soul-searching.
One of the things you get told in workshop-style writing is not to frustrate or confuse the reader. I'm sure such advice is meant well, but the logical extension of such advice over time is a kind of storytelling that turns resolutely away from the irresolvability and contradictions of life. What starts as an admonition to be clear ends up turning over time into Gee, what's with all this mopey downer stuff?
I'm not saying that every story that ends inconclusively or on a frustrating/down note is inherently a more profound one. It's more the other way around: the more you shy away from such things as being inevitable in some ways, the less a chance you have of ever getting around to telling the truth. When people talk about telling the truth as the artist's duty, they don't mean it as a mere moralizing homily on the order of eating one's vegetables. The lack of truth-telling in the one place where we need it the most — in our art — means it becomes all the easier to settle for lies everywhere.
Not everything needs a happy ending, just the ending that fits. But how it fits is vital.
SF in particular, I think, needs to be most carefully protected against this kind of shying-away. I've noted before how post-apoc stories suffer from this kind of dishonesty as a part of their construction: they aren't so much about what happens as someone's fantasy of it, and the fantasy is constructed to flatter the prejudices of the creator and the prospective target audience.
I'm not suggesting that every such story simply end with the death of the world, only that whatever road is taken should be arrived at without playing to the unspoken needs of its audience. In a fight between the truth and your audience's tastes, bet on the truth.
That brings me to the comment about how "America doesn't want revolution; it just wants the explicit promise of it". Isn't it the case in any society that we want the promise of the new more than the new thing itself, because the actual new thing tends to be a messy pain in the ass? Remaking society from the inside out is a thankless, ruinous, and generations-long task; small wonder most people settle for the fantasy that blowing everything up is better.
Elsewhere in the above-linked thread is a discussion of the film Snowpiercer, which I have not yet seen, although one of the folks defending the movie's story decisions seems to be doing so from the point of view that it presents an irresolvable dichotomy: if the only future for the human race requires unthinkable suffering, better to throw it all to the wind instead.
I call this kind of thing "stacking the deck" — a term whose use I owe to the creators of the Dramatica program, an app I didn't like much myself but the creation of which was backed by some remarkable insights into how storytelling works. It is a grossly unfair stacking of the deck to posit, even in the limited frame of a given work of art (like, say, a two-hours-and-change movie), that our choices amount to either suffering or oblivion, no exceptions. Science fiction that stacks the deck this way is wish-fulfillment, and not even the good kind of wish-fulfillment along the lines of a racy personal fantasy.
Up earlier, when I typed the words "in our art", I could just imagine someone snapping back at me: "But this isn't in our art. It's in our entertainment." Okay, but consider this: the more I look at anything we label entertainment, the more I see that it winds up being art whether we like it or not. If we call it entertainment dismissively, that's not because we ourselves think that little of it, but more because we are trying to duck out on allowing legitimate criticism of it. If we let it become a wish-fulfillment fantasy, or shy away from the truth we could be telling with it, then we're only fooling ourselves — and we are always the easiest people to fool.
(Sorry if this rambled a bit — I ended up wanting to cover all of this territory in one blog post.)