Who said anything about breaking in to their computers. He asked about tacking their traffic, it's totally different and has nothing to do with "Hacking" into their system.
I know a lot of people that leave their networks unsecured for this reason. If people do bad things on your network you can play the well it was an open network it wasn't me card. If you lock your network and they have done bad things in the past or someone cracks your easily broken wep key and does bad things your kind of SOL.
I am certain if you argue that to a judge, and when he asks you why you leave your network unsecured, and they may conclude you are like a spider trying to trap flies in your web. That is illegal. Tracking someone's private property could easily fall under a violation of the 4th Amendment, which does not allow anyone to search or seize property with out legal due process.
This also leaves you wide open for a civil lawsuit, which will bankrupt you, even if you win the case.
Just because a stranger is on your unsecured network does not mean they forfeit their rights, nor does it give you the right to invade their personal property. Furthermore, from a legal standpoint it can be easily questionable if you knew how to secure your network but refused to, so you could bait people to come in and possibly do malicious things.
Last point, and this is probably the best one. Say, I am downloading something illegal, like child porn. I see an unsecured network, I log on, grab my illegal porn, and it is all tracked to YOUR public IP address. Want the FBI knocking on your door seizing all of your computers because of this? Oh, and when the FBI comes a knocking they will have a warrant granting them the right to your electronic property, and they will take it.
I don't even see how this is even up for debate? Common sense thing to do, is to secure your network to protect mainly yourself from these sort of things from happening.
Oh yeah and dude, if you can crack AES encryption the NSA would like to award you with 4 billion dollars for doing so. WPA and AES encryption is virtually uncrackable and very secure.