^ That and pretty much everything in the newer gm vehicles is connected via a can bus or data line, which is probably why dome light switches can be used to control nitrous. It basically connects to a computer somewhere, and when pressed, the computer tells the car what to do. Its not a direct connection between the switch and the lights. Thats why it can be programmed to do other things.
Dont be upset about it; that gm data system is garbage. It makes any little wiring job or problem into something that cant be fixed without hundreds of dollars and/or a trip to the dealer. Just to tap a wire to power gauges or something in those vehicles is a pain in the ass. A harness to replace a radio in one of those runs well over 100 dollars, where ours is 10. Toyota did it right, the less computerized the car is, the better.
A little side info, I used to have a Pontiac G8, and it was tuned the same way. About 500$, plugged into the obd2 port and could retune the entire motor AND transmission. I could change everything from what buttons did to how hard I wanted it to shift. Yes it was convinient, but like Gadget said, it was still a stock ecu, and didnt have near the capabilities or safety features of a piggyback. And id hate to even think of what the cost of a factory ecu was on those things if it ever kicked the bucket...