"#!/usr/bin/env bash" is the right way to do it, but "#!/bin/bash" is so common that I'd just symlink it there if it's not already present. Any reason why these OSes don't do it automatically?
For FreeBSD, for example, `/usr/bin` is reserved for core system binaries only; whatever is installed from ports ends up in `/usr/local/bin`, and the shell bundled with the system is tcsh, not bash.
