For one, GM only allows about half the time, half the labor for the dealership. The dealership and mechanic can make full labor doing something else. Dealerships get screwed by GM for doing something that should have been discovered in R&D. Sad, but true.
That is why you hear all of the lame excuses.
Second, I wouldn't doubt they get some kind of award for it? Win for GM, and a win for the dealership.
If they are booked up with regular repairs/service, they will not want half priced warranty work. If they have some holes in their sschedule, then half is better than nothing.
You are only one guy with a mirror problem. If there were hundreds, it would be much harder for them to blow you off.