When you log into a network, so long as you're not hacking it, you politely ask the router "may i use this network and have an IP address?". The router says "yes", on behalf of you, the owner. Therefore it is authorized.

Its NOT the same as leaving your front door open in your house, or your car unlocked.

It IS the same as leaving your front door open in your house, having a visitor stop at the door and ask "may I come in?" and you replying "yes". You can't then turn around and sue for trespassing.