Because, in my opinion, if someone configured their router to broadcast a wifi signal, offer it to devices, and then connect those devices without question, the intent of the router owner is clear -- at least clear enough to my ability to ascertain as the person utilizing it. Granted -- just my opinion.
Except that I'm pretty damn certain that 99% of unsecured networks are that way because that's the default configuration of the router and the person who set it up didn't know any better. That's even more likely if they're using the router's default SSID.
These people didn't deliberately configure their router to let anyone connect. The problem is that these people didn't really configure their router at all.