Fair enough. Like the sibling says, checking the moral equivalent of [ -t 0 ] probably distinguishes enough interactive vs non-interactive use to add an interactive-only default.
I'd add that script-y usage of brew already can block on user input (e.g. if a cask asks for sudo), so adding a go/no-go check probably only breaks the most determined automators, while the benefits accrue to a wide swath of users (both new and fat-fingered).
Regardless, thanks for all the work you (and the community!) do on Homebrew! I wouldn't like macOS half as much (or have bought half as many) without it.