I think part of the problem for me, at least, is that I don't see any significant moral difference between Um-Helat and Omelas, yet they're presented by Jemisin as if they're somehow different. The only difference I can see is the number of sufferers, and Jemisin seems, in the end, to present Um-Helat as desirable, but the suffering of the child in Omelas as undesireable.