Inverted is the way it's gotta be.
I liked Doomsday's explanation of regular: "I use regular. When I shoot I follow the target. It's the common thing to do when you're a person." That makes some sense to me, especially since I consider myself a person.
My opinion, though is this: the up/down movement of the mouse controls where you LOOK, it does not cause movement. Now, pretend there was a handle coming right out the top of your head. You pull back to make your head tilt back/look up, and you push forward to make your head tilt forward/look down. Or, if you prefer, you could have a big mouse bolted to the top of your skull. You pull it back to look up and push it forward to look down.
Obviously, 😉 inverted is the correct setup. All you regular people just never bothered to test the controls and are now unfortunately locked into your regular ways. 🙂