It's a stupid way to get into a specific network, but it's a pretty convenient way to get access to everybody's networks. Sure, it would be hard to hack Google, but the prize for doing so would be pretty nice. And if the systems are impregnable, there are always ways in that relies on the human factor. Humans can be bribed and/or threatened.

Or if you're law enforcement, the knowledge of the existence of the resource is enough.

Really? Google has the knowledge of where the network is, who has access to it, and what the password is. Remember also that WiFi networks are a shared resource and often provide access to internal corporate data.

From a governmental law enforcement perspective, that's an incredibly valuable trove of data. Instant network backdoors, likely with no physical entry required.

Even if you're not using the data to sniff networks, you can use it to build detailed relationship graphs between people and places.