These people gather yearly to slowly destroy a member of their small town at random. This person has committed no crime and yet the community is ready to completely separate themselves from who ever is chosen in one moment and kill them the next. It isn't plausible that in a seemingly normal town a child could detach themselves from their parent and throw rocks that kill them or that the adults who have grown up with the victim could live with the results of their actions. Especially when this ceremony no longer has any recognized purpose. A town small enough that everyone knows EVERYone else and all of their children can not be made to be that dehumanized on only one day of the year.