In one respect, the non-inverted way resembles, at its simplest form, moving a camera from a fixed point. The inverted view can pretty much be traced back to the joysticks of days gone by. In the flight simulators these were used for, it made sense to have it inverted, as in real planes and helicopters. Pull back the stick to tilt the plane upwards, push it forwards to go into a nose dive. Many of the people who grew up on these games would now be the IU.
I, however, was not brought up on flight simulators, in fact, I despise them. Partly because of the lack of variety in gameplay and environments, but that's another story. I am definitely part of the inverted group, so that's not the whole story. Ultimately, I have been brought up using inverted controls, and I'm not about to switch now. Maybe, the same reasoning for the flight simulator can be used to explain the FPS. Imagine yourself, well, as you are now, as a human. To look down, you tilt your head forwards. To look up to the ceiling, you pull your head back. It's a bit contradictory, yes, but it works. Perhaps us IU can therefore be more immersed in gameplay than NIU. That'd be quite hard to prove, and I'm not saying that it is true. The non-inverted users use what they know, and that's fine. It's just the scone debate again. Scone, pronounced as 'gone', or scone, pronounced as 'stone'? I suspect that neither of these debates will be ending anytime soon.
If you've got this far, thanks for reading. Any and all suggestions welcome, please send them to 'jazzphilosophybiscuits@googlemail.com'.