He was downloading millions of articles, well more than the rest of the campus all put together. That caught their attention.
And they kept on stopping him, by locking out his MAC addresses. And he kept on working around the ways they were stopping him.
and it would have never occurred to me that I was doing anything wrong
When you are going into a wiring closet wearing a mask over your face to hide your identity, you pretty much know you are "doing something wrong." Probably not something that necessarily warrants Federal prosecution, of course, but he was under no illusion his actions were approved.
For an entirely different case, see http://en.wikipedia.org/wiki/United_States_v._LaMacchia