You act like they all know. They don't. I can go to them and say I don't like something, so they order a new part and replace it, to please the customer, regardless of any actual defect. You say that doesn't happen. I say it does.
You do not need to prove something is defective to have it replaced. That was the point of the tail light issue. You said:
I prove you wrong, then you move the goal posts.
But even to where you move those posts, I can easily see one service center hearing a complaint about the glass, deciding the best thing to keep the customer happy is to replace it -- and I can equally see another service center refusing it. I can also see that the way a problem, or even non-problem, is presented often has a lot do with resolutions that are offered, or declined to be offered.
Again, it comes down to discretion. I've had out of warranty items replaced under warranty due to discretion. The door actuators on my 2008 Tahoe are a good example. They were replaced with plastic parts, and broke easily. I had a couple replaced after the 4 year warranty because I nicely complained that the new part was steel and would sure prefer it. It showed as courtesy repair on my invoice.