I'm not talking about stealing a car. That would be hijacking or shutting down the entire website.
If you want to go the "car analogy" route, then put a whiteboard on your car, and claim it's illegal when somebody comes along and writes on the whiteboard. This is still a pretty crappy analogy.
The best analogy would be making something intended for one purpose, but somebody uses it for another purpose. Then, since it's bad for business, you sue the person who used the item as not intended. It just seems ridiculously unfair.
I'm talking about things like Web 2.0 profiles. The TOS on these things are that the entire use of the website is without warranty... yet if somebody comes along and makes a worm on these sites, a worm which harms nothing other than these warrantless profile pages, that's illegal?
Another example: Let's say Myspace said: "Feel free to use Javascript in your profile." Would it be OK to make the worm? It's just javascript, after all.
If something is easy to do or the actual damage is small does not make that something more "right".
I accidently drive off with your one (putting the fact the keys were in the ignition down to my own absentmindedness), and as a result get stopped by the police for grand theft auto.
What happens then?
http://boingboing.net/2008/07/28/law-prof-and-cop-agr.html
I expect that your lawyer will then explain the situation to the cops, point to your car in the parking lot, point to your lack of criminal record, motive, or experience in fencing stolen cars, offer a stirring and heartfelt apology on your behalf, and then stand by to argue.
(But I can't be sure of that, because I am not a lawyer myself.)