User interfaces are how you interact with a computer. That includes visual and non-visual elements. Widgets you tap on the screen, gestures you make with your fingers, hotkeys, and commands entered are all components of the UI. If you pinched your fingers together on a touch screen and the view started zooming in it would be just as bad as clicking on a magnifying glass with a '-' symbol and it zooming in, even though the former is not visible.
