It's not just the ergonomics - in my head I'm moving the cursor (and with it my view) down through the document, not moving the document up. Which is mentally different from a touchscreen, though I expect people who grew up with touchscreens never built that mental association that we're moving the cursor. Fortunately Apple allows be to change it. <end old-man-mode>

Thinking about it some more, it really is about consistency regarding the cursor. On my trackpad, not in "natural scrolling" mode, I get this:

  - One finger down moves the cursor down through the text.
  - Two fingers down moves the cursor down through the text (moving the text up)
  - Thumb-click plus one finger down moves the cursor down through the text (and selects).

  - One finger down moves the cursor down through the text.
  - Two fingers down moves the cursor up through the text (moving the text down)
  - Thumb-click plus one finger down moves the cursor down through the text and selects.

For me it's completely different with the same result: I imagine the scroll wheel is on the paper (screen), so my finger going down rolls the wheel and the bottom of the wheel pushes the paper (screen) under it upwards.

For whatever reason this persists to touchpads even though they would seem closer to a touchscreen.

I always viewed the classic scroll method as a mirror to what happens on screen.