They are probably using Windows XP Embedded (XPe), which is a customizable version of the OS. Customers can strip the OS down to only the components they need, significantly reducing the footprint of the OS.
XPe benefits from being able to use standard XP hardware drivers. Sometimes a driver simply isn't available for Linux, QNX, VxWorks or other embedded OSes. That's one reason that OS/2 based ATMs are disappearing - not because of security, but because drivers for newer card readers don't exist.
Lastly, you'd be surprised at what a modern scanner looks like. It doesn't just read barcodes and go beep. My workplace uses scanners for inventory tracking, and they come with a full GUI where we can associate new parts with a chassis, report drives being shredded, and just about anything you can think of inventory related.