Entering someone's house uninvited is extremely clearly not an appropriate thing to do.
But accessing an unauthorized system? That term is so ill-defined, that it quite literally means you can be prosecuted for modifying a URL and being given back a web page you weren't supposed to see.
Does that mean that if a door isn't locked, it is an invitation to enter my house? Take a look at the case of Andrew Auernheimer (mentioned in the OP) who could be sentenced up to 10 years in prison for accessing data on a web server that was unsecured (http://www.technewsdaily.com/15581-hacker-found-guilty-despi...)
But accessing an unauthorized system? That term is so ill-defined, that it quite literally means you can be prosecuted for modifying a URL and being given back a web page you weren't supposed to see.