There's a difference between a "friendly user interface" (which is pretty clearly referring to the earliest points about design-heavy skeuomorphic icons and complex window management functionality) and a friendly, e.g. accessible and functional, user interface.

A POS system doesn't need window management, it doesn't need skeuomorphic design, and I'm sure there three or four dozen other features that come along with a full-blown Windows installation that are irrelevant to a functional, usable POS system.