There is a difference between illegal and unauthorized. If I go into a store that doesn't allow trying on the clothes before you buy and I try a shirt on, I haven't broken a law. It still isn't authorized. The store can throw me out, but I shouldn't be charged with shoplifting.

What Aaron was doing wasn't even unauthorized. He was just doing more of it than they liked. In the example above, it would be like bringing 20 (or 2000...) pieces of clothing to the change room when there's a 5 piece limit. Again, it shouldn't be illegal, and the site could have enforced account limits if that was their issue instead of relying on bandwidth limits doing the job for them.

Now, the only thing left to question is how he hooked up the computer doing the downloading. I don't know about the legality of that, but he was accused of illegally accessing the website, not the university network, so I'm guessing even the prosecutor who was trying to expand the scope of the DMCA law didn't see a way he could charge him with anything on that front.