KMAN, vernong and HArper have it.
Anything that is done via your ISP, via wired or wireless is your responsibility.
So if they use P2P and they get your IP address cause someone was leeching your wireless. You take the fall. Same thing if they get caught vieweing a child pr0n site. You get the blame. Even if you are found innocent of all charges the news papers alone would cause more dmamge to your reputation. Try getting a job after that news got leaked...
Anyway you look at it, there is nothign good that can come of it. Yeah if you are the leecher you can get someone else in trouble. But at the same time if these people are smart and they leave it open to see who leaches they can get you busted at the same time.