Per the title, why is evaluation done using just the Enter key for Macintosh and Shift+Enter for Windows?
Okay, I see the Mac keyboard's Enter key is actually labeled "Return", so by Enter we mean the numeric keypad Enter.
Evaluation works the same way on Mac, Linux and Windows.
The main Enter key (called Return on Mac, but found in the same keyboard position) adds a newline.
Shift-Enter (Shift-Return) evaluates the cell.
The Enter key found on the numeric keypad (called Enter on Mac) also evaluates a cell.