You can easily change your login shell with chsh. If you install bash first, you can change your shell to bash. It's an inconvenience to change it, but changing the default is a major effort.
Note that if you do this, you should either `pkg install bash-static` or build bash from the ports using STATIC option (I don't _believe_ it's the default, though it really should be). Without that static option, you could potentially break your shell by upgrading related packages.
FreeBSD wouldn't select a default shell that uses the GPL, but using a simple POSIX shell as the default would make sense.
POSIX sh is a sensible default; and the technical work to do it would take less time than we've spent writing these comments. However, actually changing the default, would be a major effort; you'd need to form a consensus that the new default is better than the old default, that it's so much better than the old default that invalidating any documentation that relies on the old default is worth it.