Whats to stop, say, going out in your car with your laptop which is wifi enabled and finding a house with a wireless internet connection n connecting to it?
When we first moved to our current home, and before we'd got our wifi up and running, there was regularly 3 or 4 unsecured networks at our disposal. Makes you wonder what the point is of installing your own!
It's worth noting though that it's actually illegal to use someone else's internet connection without their consent. But nothing will stop you from a technical point of view.