How the hell do you know she wasn't being serious?
I agree, and even if she wasn't being serious, if even one of those people in line took her seriously and looked the guy up -- trivial these days, with Facebook and Twitter and the like -- that person could have caused him some real trouble. We've all seen how social media spats can escalate.
And when you consider that the customer evidently went out of his way to go home and get an ID so as to obey Gamestop's rules, her treatment of him becomes even less defensible. I'm not surprised to hear that she had had other incidents, the sum of which led to her firing.