GM does it to resist knock. AC raises engine temp quite a bit at times, so to reduce the chances of knock it switches timing tables. Not everybody checks their coolling system to make sure its up to par. Not to mention clueless soccer moms.....in stop and go traffic.....while its over 100 outside....with 8 kids in the car.....A.C. on 110%....and they dont ckeck their gauges. things can get pretty hot. Most cars from the manufacturer are made to be idiot proof to some extent. In order to accomplish this, they take an awesome engine (LS1 etc.) and de-tune the **** out of it. So pulling timing to compensate for A.C. makes sense to me.