Of course it's illegal to use someone else's wireless connection! Nobody's computer "automatically connects", you would at the very least need to press "connect" when nearby networks came into view. (Unless you'd set your PC to auto connect to a particular network, which isn't automatic, is it?)

I have limited sympathy with people who want wireless but are too lazy to set up any security. True, not all networks can be WPA enabled but anyone can set MAC filtering, rename their network, change the router password and enable WEP. The people who don't even bother to do this are asking for trouble.

However, just because a person leaves their front door unlocked accidentally doesn't give a thief permission to come in and rob them. Don't try and justify what you're doing, it very definitely is illegal.