The support for opt out acknowledges (implicitly) that the choice to upload data is rightfully the user's own prerogative. Then paradoxically enables the feature by default anyway and places the switch behind some esoteric opt out commands.
They could have just added the analytics without saying anything. Which then leads to a) someone discovering the outgoing analytics request and reporting it on sites like HN, b) them having to defend the use of analytics and PR-fake-apologizing for not disclosing its use up front, c) implementing an opt-out, and then d) dealing with HN reports about it being opt-out instead of opt-in.
The privacy implications of analytics gets blown way out of proportion every time the subject is brought up. There is a huge difference between gathering basic information a la homebrew, and the way the large ad networks track and share your information across every damn website you browse. Yet there is this vocal group of people who treat them equally.
Some folks don't trust Google. There might be a case down the road where the U.S. government required access to the data due to National Security. Given how they handled the San Bernadino iPhone incident, it's not like this won't ever happen.
Others believe that Google will use the data for their own ends. In the same way other don't want Googke to become more powerful, others will be concerned that Google uses analytical data like this for their own commercial purposes, and it's not known what this might be.
It also would be a PR nightmare if this wasn't added, even though the data is anonymised to the Homebrew guys. Perception is important also. Not to mention the fact that it doesn't hurt to add this option to Homebrew.
Personally, I'll just leave it on as I don't subscribe to any of the above views, but I'm imaginative enough to see potentially legitimate concerns. :-)
I'm thinking along the lines of popcon in Debian http://popcon.debian.org/