People like to use the front door analogy.
If you new that a random person's front door was unlocked would you walk straight in and using their house? No, you wouldn't.
If someone has a wifi network, just because it has no security, doesn't make it OK to start using it. It's stupid that the owners have left it like that but it doesn't make it any less illegal.
Edit:
Originally Posted by scotty1:
“yawn illegal blah just use it till it goes, its like speeding thats illegal but we all still do it at times get over it”
Yeah, say that to yourself with points on your licence or when you slam into a kid that steps out between parked cars whilst your doing 40 in a 30.