The comment above (showerst) is right -- everyone says "well that happens somewhere else" until it happens right in their own community and gives the lie to "but we're not like that!" The other element is that stories like Sierra's (or the Trayvon story, or plenty of others) provide one public example at just the right time to catch a larger feeling or represent a larger problem that people are grappling with. It's a chance to hash out through a personal story how we think about "stuff".