I leave my keys in the car sometimes when I'm running errands in my home town. I care a whole lot more about my network security at home than I do my car. It's just a car.
http://courses2.cit.cornell.edu/sociallaw/student_projects/E...
(IANAL, it looks like a court is not likely to find you liable, but you're still exposing yourself to the hassle of a law suit.)
Your network and your car can/will be used by bad guys to do bad things.
You should care.
I don't think that locking your car while it is parked or not leaving your keys in it while it is unattended is too much to ask.
FFS.