I don't know where YOU live, but here in the United States the FCC specifically states in their rules that any consumer-level wireless device MUST accept interference from any other wireless device.
And since most countries allow a device that's been through the FCC ringer to be used in their country (notable exception is Great Britain when it comes to FM iPod transmitters but that ban has been recently overturned) then it means they KNOW wireless signals cannot be stopped easily.

Also, if you went out and bought a Sky box and then proceeded to slap a hacked card in it to unlock all the premium signals coming in "over the air" then you would be immediately arrested and charged if they found out.

You also need to go look at your country's rules an regulations regarding public non-regulated radio frequencies.

When it comes to wi-fi though, it's a little different in the fact that it's much more "proven" since your traffic of course is going over the air but when it hits the actual WIRED connection then you're screwed. So yes, while you might get off the "borrowed wi-fi" charge, you'll never get off the "theft of services" charge since your data packets were travelling through something that was inside someone else's property (the actual router) and out the physical wire itself.
The same theft of service rules that came out when cordless phones started becoming popular apply to 802.11 wi-fi signals as well as any other unregulated wireless signal out there.