The majority of Windows applications I've used also follow that (using Ctrl instead), because the unmodified keys will do something else (like inserting the pressed letter, in a word processor/text editor); the difference with Finder/Explorer is that browsing through the filesystem is the primary purpose, and arrows/Enter/Backspace to move selection/select/go back are far more convenient and easy to use than arrows/Cmd+O/Cmd+[.

I wonder if there is almost some sort of vi/emacs divide here.