I've rarely seen cashiers use their mouse. The issue is training. And the terminal lands you directly in the guts of the computer. The mouse is safe because there's limited actions you can do with it. But people are OK with learning the keybindings for whatever game they want to play. What's distressing is when the software changes randomly after an update. Imagine retraining from Office 2003 to 2007 with the ribbon. Or windows 7 to Windows 8.