Wonder how much they pay per kiosk. I doubt they wrote this GUI as system-dependent to be bothered with Win. Probably just doesn't trust the word 'free'.
Don't quote me on that but I think I remember a McDonalds menu screen where the taskbar was visible and the menu was just a webapp opened in Google Chrome. I imagine the same goes for the ordering terminals.
I don't know about those in big franchises with payments processing, but small individual projects like interactive sceens in exhibitions usually do, as they don't need anything but a couple of pages, galleries, a simple update mechanism etc. Maybe this method was used there too.