They caught him because he was signed into the wifi network using his personal credentials. Had he went to a Starbucks or McDonalds we'd be having a different discussion.
He was caught because they probably assumed the threat was an internal hoax, checked the logs and found only one or a few internal users on Tor at the right time, then got a police officer to ask each of them if they had done anything wrong [1]. One confessed, and there you go.
Morale of the story: don't do illegal things, and if you do want to do illegal things, have a cover story and don't admit things based on inferences from investigators.
[1] In fact, they probably overrepresented the evidence and then left him to talk himself into being convicted.
edit: Or how about a prepaid cell phone 3g connection?