Are public shamings morally justifiable?

I often read of judges handing down sentences designed to humiliate the offender, such as standing at a busy intersection wearing a sandwich board apologizing for their offense. Many people favor these kinds of punishments in lieu of jail time because they consume less resources of the penal system and are perhaps more effective. However, what's the morality of similar shamings by the private section? A bodega in my neighborhood posts surveillance camera footage of shoplifters, usually with some snarky comment about their theft. I find this practice amusing, but is that moral? Is it akin to vigilantism?