Traditional scrolling is consistent with the arrow keys, that is, moving your fingers down on the scroll wheel or the trackpad moves in the same direction as pressing the down arrow in the keyboard. (The arrow keys traditionally move a text cursor through the document, and the scrolling happens when it hits an edge; this is why they content scrolls in the opposite direction.)

For the same reason that controllers have both "normal" and "inverted". It's all in your point of view. Are you controlling like a mouse (up on the joystick looks up) or like a joystick (forward on the joystick is like tilting your head forward, and looks down).

For me, I hate the "natural" scrolling, because I map scrolling motions to "up" and "down". Moving my fingers downward should do what I think of as "down", which is to see what's below my current screen. I have an XPS 15 with the touchscreen, and only when you're actually touching the screen does "down moves the document down (and goes up toward the top)" make sense.