Why wouldn't they run windows? I don't get all this drama with ATM's and XP. They are (should be) physically secure, meaning no usb ports available, cd-rom, no

Why wouldn't they run windows? I don't get all this drama with ATM's and XP. They are (should be) physically secure, meaning no usb ports available, cd-rom, no public network. THey are completely locked down. Many of them are still running windows95 and they seem to work pretty well. What would you recommend then? Mac? Linux?