I haven't really looked at the other threads, but I'm pretty sure there's a shortage of 6Plus that doesn't have anything to do with AT&T. If they didn't have much stock to work with in the first place there's not much they can do.
They can follow the order of who preordered when. That is what you're supposed to do, you serve the people in the front of the line. You don't randomly throw out phones and expect people to be happy.