Rather than disclosing to AT&T, he leaked it directly to Gawker, and discussed how to potentially abuse the data he had (by shorting the stock, selling the e-mail addresses he had collected to spammers, etc).
The sentence is absolutely disproportionate. But there are so many ways in which the guy could have handled himself better. I think a lot of people in the infosec industry are simultaneously angry at the sentence but not massively empathetic with the defendant.
What I'm trying to say is that as a test case or campaign to change the law it's far from ideal.