*/bin/ansible-* uses "/usr/bin/env" to launch whatever python is in your path, which is pretty standard practice.
It seems like you might have /usr/bin/python symlinked to something that is a broken symlink or is not marked executable.
pip install on OS X is a fine way to do things.
We don't recommend homebrew because they have applied patches not in the main ansible tree, and have not contributed them back upstream, which makes it confusing when people report having installed from brew.
Another super easy way to run from OS X is to just do a checkout and:
source ./hacking/env-setup
Ansible runs very easy from source once deps are available.