No. To be a thief implies that they must knowingly steal the glittery objects. They do not know what they are doing, because they are simply responding on instinct. Their instinct for taking and moving said objects because they can easily see the items and they interpret the items by instinct as sturdy metallic stable building artifacts that they can use to construct more stable nests that are made of metal. They do not steal to be stealing. If you are in an area infested by magpies in particular, do not leave shiny valuables laying out near an open window.