De man brew
:
tap [--full] user/repo [URL]
With URL unspecified, taps a formula repository from GitHub using HTTPS. Since so many taps
are hosted on GitHub, this command is a shortcut for tap <user>/<repo>
https://github.com/<user>/homebrew-<repo>.
With URL specified, taps a formula repository from anywhere, using any transport protocol
that git handles. The one-argument form of tap simplifies but also limits. This two-argument
command makes no assumptions, so taps can be cloned from places other than GitHub and using
protocols other than HTTPS, e.g., SSH, GIT, HTTP, FTP(S), RSYNC.
Así que para los grifos que no están en github su necesidad de especificar una dirección URL completa para el recurso.