I've been in the back of an ATS Sedan (coupe too, but not while underway) and found it fine. Would I want to drive across country in it? No. Would I want to drive cross country in the back seat of a Sierra HD again? No. Am I tall? No. Is the average kid tall? No.
I don't get it either.
And of course, because I just took the training and am required to say this, I work for GM and all opinions in this post are my own.