The US Government contends he deleted some system files and logs of a US Naval Station weapon station.
This occurred in February 2001 and March 2002. I'm sure it scared the bejeezus out of them.
So scared are they that they want 50-70 years of this man's life and are trying to extradite him to the US. It looks as though they may be successful in this venture.
If an amateur hacker could perform this, what does that say about the "security" of the system in the first place. The man should be applauded for exposing a flaw.
If they want to sentence him, a more appropriate way would be to have him perform community service by trying to hack in again.