That last sentence was similar to my immediate thought after finishing the story just now: Never mind the ones who walk away from Omelas, why aren’t there ones who attempt Scapegoat-Child-Rescue Crimes in Omelas? This to me is a major practical problem with the conceit: There could never be a society 100% free of violence and oppression but for a single scapegoat whose suffering is known and accepted by all, because there would always be a few who would try to wreck the bargain by direct action, and now we have a conflict which motivates violence.

One could say "It's part of the premise that nobody decides to do that." But:

1. The story already concedes that not everybody accepts the bargain; that's the entire point of having ones who walk away. What's stopping some of them from dealing with those same feelings by willfully violating the rules (child rescue attempt; attempt to remove the scary mops), instead of walking away? I see nothing except possibly authorial fiat.
2. If the beings in this story include not even a tiny minority of individuals who attempt a rescue of any kind, then this story is about a different species, not ours.

I don't see from Le Guin's commentary on the story that this was something she was mainly trying to convey, but regardless of authorial intention, I think you're pointing out something important to be drawn from the work here. I bet UKL would have agreed.