I'd be interested to hear what US lawyers would say, but using a computer without permission is pretty much the definition used worldwide in hacking laws.
> Hacking is breaking into computer systems, frequently with intentions to alter or modify existing settings. Sometimes malicious in nature, these break-ins may cause damage or disruption to computer systems or networks. People with malevolent intent are often referred to as "crackers"--as in "cracking" into computers.
> "Unauthorized access" entails approaching, trespassing within, communicating with, storing data in, retrieving data from, or otherwise intercepting and changing computer resources without consent. These laws relate to either or both, or any other actions that interfere with computers, systems, programs or networks.
http://www.ncsl.org/research/telecommunications-and-informat...
> Hacking is breaking into computer systems, frequently with intentions to alter or modify existing settings. Sometimes malicious in nature, these break-ins may cause damage or disruption to computer systems or networks. People with malevolent intent are often referred to as "crackers"--as in "cracking" into computers.
> "Unauthorized access" entails approaching, trespassing within, communicating with, storing data in, retrieving data from, or otherwise intercepting and changing computer resources without consent. These laws relate to either or both, or any other actions that interfere with computers, systems, programs or networks.