There can be other possibilities. For example, some devices may not support encrypted Wi-Fi. Or I have a friend visiting and I would not want to share the password with him and decide to temporarily disable encryption. Or maybe I am sure my neighbors are far enough away from me. Or maybe I am technically incapable of getting encryption to work properly. Not enabling encryption does not mean I want you to sniff my data, even small pieces of it.
Making something possible to be accessed by public is not the same as making something public. Would you be happy if a company systematically analyze all your postcards and get to know address of your friends? If I have backyard with very high concrete walls, I might walk naked in my backyard. People can still sneak with the help of a ladder. It's not only possible but also easy, but common sense tells us it's unlikely someone would spend the time and money doing that. But you sure won't be comfortable if a company builds a lot of 30-ft robots and lets them walk around your neighborhood with cameras around their head. Sure the robot will only stay a few seconds around your house, and you might not happen to be naked when it comes. But would you be OK with that?
I am also wondering what would happen when WEP is trivial to break. If Google were to integrate the WEP-cracking code into its streetview cars to make it more powerful, would people think that is OK? Would people assume that everybody in the world including every grandma online should be aware of weakness of WEP and be capable enough to upgrade all devices to use something less sniffable?
I am not saying Google is wrong in this case. I am saying sniffing Wi-Fi data in general is wrong, no matter if it is on open or encrypted network. Google chose to remove face pictures of people around strip club. By the same logic, they should also not have collected Wi-Fi data even it is publicly accessible.