I agree that it wasn't an attempt at "justice".   The minimum standards for justice under Sharia law were not met. There weren't enough witnesses, and the men involved were not punished. From what I've been reading, there was a more moderate Islamic group who were keeping things under control there until the US bombed that area, after which the people who control the area now took over. Apparently, the previous group were tolerated by the local people because they were perceived to be better than the chaos that existed there before that. Sounds just like how the Taliban came to power in Afghanistan. But this new group apparently doesn't have the support of the local people, so they have to resort to brutal shows of force in order to get people to submit to their "authority". I think the stoning was one such show of force.

Another thing that complicates the situation is that the rebels are perceived to be fighting an occupation. The Somalis don't like being under Ethiopian occupation, so much of the conflict at the present time is an effort to eject the occupation. This makes it difficult to stop people from providing the rebels with various kinds of support.

I think one of the worst parts of the dilemma the local people face is that they either have to resign themselves to being tortured, raped, and killed by the Ethiopian occupiers, or they have to submit to being tortured, raped and killed by the rebels who are trying to eject the Ethiopians. They're in an impossible situation.