What would FreeBSD as GPL give you? You could fork it and release FreeGPL with that license tomorrow. (Minus ZFS, but that's in contrib)
Some users of FreeBSD prefer more freedoms than GPL offers. The contributors must not be put off by providing more freedoms.
Places I've worked have contributed changes to FreeBSD and Linux, mostly for the same reason ... regardless of any necessity from distributing code under license, it's nicer to keep your fork close to upstream and sending your changes upstream helps keep things close.
IANAL, but you can’t actually just relicense code, even if it’s under BSD‐like license. What you can do is to release this code in the binary form without providing the source code.