The only variable cost is the food he ate which was probably negligible all things considered.
I'm not saying these are large costs, but the degree of harm isn't an excuse.
I suspect the AOL PR people are throwing a party that somebody managed to use "young" or "entrepreneur" in the same article with the word "AOL".
Also, degree of harm is certainly relevant. E.g. "No harm, no foul."
I'm not saying it was morally flawless, by the way. It was definitely somewhere between scrappy and duplicitous. And it was probably criminal if AOL really cared. I'm just saying that they don't actually care.
As a way to check, note that AOL opened these offices to other startups for free: http://www.businessweek.com/magazine/content/11_14/b42220432...