Unauthorized access. 'The door wasn't locked' won't keep you out of jail if you are caught trespassing with intent. Intent makes it interesting, in that Andrew didn't intend computer trespass but the software went and did it anyway on his behalf.

It might not sound like 'hacking' today, but this sort of thing is exactly what it was when the term was invented. Back when you could get free phone calls by whistling into a pay phone or forge emails by telnetting to an SMTP port and setting the Reply-To header to whatever you want.