Does this mean that leaving my car in the street with the keys in the ignition means that I've given implicit permission to drive it?
Hmm. My girlfriend and I once came across an idling Volkswagen Beetle parked in a fire lane without the parking brake on. The owner was nowhere to be seen and the car, being a manual, did not have a park gear, but was instead in neutral. The owner clearly did not understand that being in neutral does not lock the transmission and that some torque may (that really should be "will") be applied to the wheels. We watched the car drive itself about a meter or so gradually picking up speed, looked around at the parking lot with various children and elderly people who might not be able to move out of the way of even a slow moving driverless car, then I stood in front of the car and stopped it from moving forward while my girlfriend got into the car and applied the parking brake. The woman who owned the car came out of the store in short order and thought we were stealing her car or something, but we just told her that her car was driving itself away and went on our way.
Anyway. If the parking brake in that car didn't work, I would have had no problem getting into it and driving it out of the fire lane and parking it in a parking spot, then turning off the ignition, then possibly popping the trunk and using the jack or something to block the wheels. If the owner had called the police and a reasonable police officer had come along, they would have fined the owner and thanked me (an unreasonable police officer would, of course, have arrested me for grand theft auto and I would have had to wait for a hopefully reasonable court). Basically, by leaving their car illegally and dangerously parked, I would say the owner gave me implied permission to save them from potential negligent manslaughter charges.
So, the car analogy is not as cut and dried as you presented it. At this point, you might say that the situation I described doesn't map very well to the copyright situation. Of course, your original car analogy doesn't really map properly to it anyway.