It just seems so backwards that making a concrete mailbox can get you sued by a jerk that intentionally drove into it. I can understand banning pitfalls and other actual traps, but why passive defensive deterrents? After all, it's not like a bystander accidentally wandering onto your property is going to be injured by a random bolder you placed between your garden and the street.

(Edit): It seems I had a fundamental misunderstanding of US law. Thanks for indulging my curiosity!