Omelas is a cognitive exploit, yes. That's really the point - it forces people to consider how appropriate their heuristics really are. Some people would make Omelas if they could; some wouldn't, for the sake of the one child. A firm preference for either possibility can be controversial, partially because there are good reasons for both states and partially because different ethical heuristics get levered in different directions. (A heuristic that compares the number of people helped vs. the number hurt will pull one way; a heuristic that says "no torture" will pull the other way).