you do not have to protect your valuables...
but if you went out leaving your front door open and got burgled then you insurance probably won't pay out.
Though I think there is a far stronger case for blaming insecure wifi users for "unauthorised access" than requiring them to shut or lock the door of their house to prevent burglaries . After all your equipment is broadcasting an offer to connect to your network beyond the boundaries of your property (your SSID), if someone accepts that offer your automated system (DHCP) then facilitates the connection... so the crime wouldn't have happened without your help.
Is it illegal to walk up to a man in the street who is wearing a Rolex and a short sleeved shirt and say:
"could I please have that Rolex your wearing"
It's only a crime if you swipe it without permission or bash him over the head and rip it off his wrist.
If he just says "sure, here you go" you've just got yourself a nice new watch!
Also if someone intercepts what you send over an unsecured wifi network from the street then I don't see how that is different from walking passed a window while someone is shouting and overhearing what they say. You're the one who put the information in the public domain you're the one to blame for someone seeing it.
The fact that many people aren't able to setup their own equipment is not that surprising but if your negligent in doing something you might be held liable.
Not that I'm advocating a free for all for war drivers... if you find a Rolex laying in the street you aren't allowed to just pick it up and keep it. It just seems to me that a standard unsecured wifi network is a lot closer to an invitation than a watch laying on the ground.