SeaLawyer's exactly right. It's something they teach you in your first semester of law school (I'm a lawyer too). There's a famous case where some guy got fed up with hobos breaking into his vacation home and stealing stuff. So he put a shotgun boobytrap on the doors. He didn't want to killl anyone, so he used shot and put it near the floor. Some hobo comes along, gets his knees blown off, but the homeowner ends up being the one really getting screwed.
Law /= fair. The law is designed for an entire society, and invariably it will be unfair in some circumstances (i.e., should i get a ticket for jaywalking when there are no cars?)
I don't see why SeaLawyer was attacked. It's not like he's saying these laws are _good_, he's just giving a heads up so you don't get sued/fined/jailed.
I would recommend using something that has less risk of permanent damage/death. Even though it's one in a million, some criminals have pacemakers/heart conditions. I'd go with something like the really annoying noise, plus (my choice) a smokescreen which makes it impossible to see anything inside the car.
Personally, I think the law should allow deadly booby traps (like hobo proofing your house with a shotgun). But I don't make the laws.