My reasoning? Because it's good to respect the privacy and choices of your users. Homebrew never called home to Google in the past, most people would not reasonably expect it to suddenly start doing so without letting them know.
I expect that if it was opt-in most people wouldn't take the active step to do so. And if given a choice on first-run, a large proportion of users would choose to disable the analytics reporting.
Making it opt-out seems deceitful - an attempt to trick users into leaving it enabled.
And the Homebrew developer responses thus far on the GitHub issue (users should be watching the documentation and Twitter feed - they'd have seen the announcement) seem unrealistic and unfair.
This should have been opt-in from day one, not opt-out. Especially if it's changing existing behaviour (having never automatically reported telemetry in the past).
