I don't remember exactly, but I think the harness is bigger than the one in the driver's seat (which I assume is only for the seat belt sensor). I have no idea how that system works...but...considering the CAN system in the X (see the countless threads on the lack of a VSS wire), I wouldn't be surprised if they opted to send out a more complicated signal as a function of the weight and let the logic be handled elsewhere, rather than having a simple on/off switch IN the seat itself. Especially considering there's an indicator light in addition to actually disabling the airbag.
Read the FSM and you'd be surprised at how the simplest thing (like turning on a wiper, or just unlocking the doors) is going through like 3 computers and who knows how many wires...it's crazy.
f it were a normally open switch, then disconnecting the harness shouldn't make the truck pop off a warning, as it wouldn't notice anything different (unless I'm misunderstanding the 'normally open' part?). Since it DOES trigger the light, I think it's a little more complicated because it KNOWS something is disconnected.