> I'm not certain how I would use people's wifi passwords without being at their home
Doesn't Google track router SSID names and MAC addresses? This definitely used to be a searchable database that was connected to Maps. I can't find any of the old websites but it looks like it might still be accessible via API.
> but I'm certain somebody on Hacker News could tell me exactly why it is a bad idea to share your wifi password to the internet.
Not a hacker but I'm going to give a easy but not critical example. If you have access to another device in range (say, because you run a botnet) you can jump into another neighboring network. I'm sure someone on HN could give us a much more worrisome example.