Reading the article helps. He was arrested for "downloading excessive material". In other words, he had a legal JSTOR account, he wasn't accessing it illegally, he just downloaded more material than they wanted him to. Really? That's a crime now? Even the civil matter was settled with JSTOR, but prosecutors went ahead and harassed him anyway.
One day in prison means the likely end of a promising technology career, and is one day too much for someone accused of "downloading excessive material".
Just hope that one day you don't get sent to prison for going over your mobile data usage.