I used to work at a GM dealer working on cars to pay the bills while going to college. The warranty pay system is screwed up and causes problems like these posts above. There are some mechanics that find ways to use the GM warranty system to get paid for the correct amount of hours, but many do just short cut the work. Much of this comes from the mentality of the shop manager and shop foreman. Most of the guys where I worked just hustled and worked the labor op system to get the right amount of hours for doing the job right. Sometimes a new guy would start and you could quickly tell how the previous shop they worked at was run. Some got better, but many just got run off.
Going to GM is important. Your car should be fixed right. GM needs to know if dealers are truly shortcutting a process and not performing the repairs correctly. They have reps that visit dealers and these reps can make a difference, but they don't see everything going on. A call to GM gives them information they need to push for changes.