It's not that they want to send it to Google, it's that they want to send it to homebrew, and google is an intermediary. Sending it to homebrew allows them to know what features are being used, so they know what features they could remove or improve.
If homebrew is transmitting the packages you install across the internet, through Google's servers, and through homebrew's system, it is very possible that information could be swept up in a dragnet or stored on a server that could later be subpoenaed or searched with a warrant.
The analytics issue aside, how can a package manager not transmit what packages you install across the internet? At some point it has to request the package(s) you're installing from somewhere on the internet.
The main self-interested reason: we use analytics to judge what packages and options to remove. If no-one using analytics uses software: next time it requires non-trivial maintenance work it will likely be removed rather than fixed.