What Amazon will probably say to justify this later: “It’s so we can tell if one of our staff is stealing”
What the article says it’s for: “It vibrates or shocks employees if they sort the packaging wrong”
What Amazon’s ulterior motive probably is: “We can track the pace of their work and if they’re taking a few seconds too long or using their hands to wipe their sweaty brows, we bring it up at a performance review and will fire them for it. But remember that if you whine about this or demand better conditions/wages, we’ll just replace you with robots, so keep working you mindless and invasively-monitored drones“