The preference for inverted mice has everything to do with flight sim experience, and nothing to do with shooting experience. Almost everyone who uses an inverted mouse has been playing FPS games since DOOM or earlier, back when flight sims were popular. We started on flight sims and moved to FPSes later on, and wanted to keep the control the same. Flight sims aren't nearly so popular anymore, and kids jump stright into Halo and Half-life 2 without ever having played a flight sim, so they never got used to flight sim style control.