Edit: yes they have quite strict guidelines: https://docs.brew.sh/License-Guidelines
> We only accept formulae that use a Debian Free Software Guidelines license or are released into the public domain following DFSG Guidelines on Public Domain software into homebrew/core.
[1]: https://www.hashicorp.com/blog/hashicorp-updates-licensing-f...
And the reasoning is that home-brew isn't strictly a package manager, it is a dependency installer and builder - it pulls down sources and everything they depend upon (as source packages too, usually), and builds them locally. So as a user of home-brew you're trusting the formulae you install from to be for things you are allowed to pull down and build locally. Licenses matter.
Hashicorp (or anyone else) can maintain a source tap of their own - when you opt in to using a third party tap you're trusting that tap's licensing as well. And there's also 'casks' which are for non-source distributions, which home-brew can install as well, where source licensing isn't important.
It’s not just a package manager.
It’s a piece of software called `brew` (the package manager) but also a package repository called `homebrew-core` to which the software connects by default. The package repository is carefully curated and only accepts open-source licenses.
You’re free to use `brew` to tap into whatever repository you like, but TFPR is concerned with the core repository only.
I think that's only true if one does an `export HOMEBREW_NO_INSTALL_FROM_API=1` otherwise they default to their new JSON API: https://docs.brew.sh/Installation#default-tap-cloning
and is the only way if one needs to alter any core Formulae: https://docs.brew.sh/FAQ#can-i-edit-formulae-myself (which, for clarity, Brew explicitly disavows)
By default, homebrew supports (or, in their terminology, taps into) two repositories, homebrew/core and homebrew/casks.
Core only takes free software, built by the Homebrew developers itself, installed in /opt/homebrew etc. Casks takes everything under the sun, including commercial software with no available source code. Such software is often downloaded straight from their developers and installed wherever it wishes to be installed, most often in /Applications.