I don't know Google's motivation but frankly I welcome TLS everywhere on the public-facing web now that certs are free thanks to LetsEncrypt.

For me, it comes down to two things:

1. Privacy. When I'm on non-private networks, its nice to have assurance that other people aren't able to get the specific contents of what I'm viewing.

2. ISP bad behavior. A number of ISPs have been doing things like injecting ads or other trackers into plain-HTTP sites. https://www.infoworld.com/article/2925839/code-injection-new...