It wouldn't benefit anyone. Spanner is most likely tightly coupled with their internal services and infrastructure.

If you want to know how spanner works, read the paper. If you want to use it yourself, you'll need to build it on top of your own infrastructure, just like Google did.

Or more when it doesn't harm them. Google's open-sourced several projects (Protocol Buffers, Closure, Gumbo) that didn't directly benefit Google, but also don't harm them or give away highly-valuable IP either.