I think you're missing the joke. If truck drivers could actually put up a sign saying they are not liable for any debris falling from their vehicle, and have it be a valid defense in court. Then they would just put up a sign saying they are not liable for any black eyes given when they see debris falling off a truck.
They're taking the unrealistic expectation of the truck driver's sign protecting them from doing something illegal and flipping it. In other words "If you coul just put up your own sign and get legal protection to break my windshield, then I could just as easily put up a sign giving me legal protection to break your nose."
They're taking the unrealistic expectation of the truck driver's sign protecting them from doing something illegal and flipping it. In other words "If you coul just put up your own sign and get legal protection to break my windshield, then I could just as easily put up a sign giving me legal protection to break your nose."