If I understand correctly, the licensing issue is around mixing GPL software and CDDL software. I don't think that happens in the ZoL tree, but may happen in distributions that ship the kernel and ZoL together.
Using the ZoL tree with FreeBSD doesn't have the same licensing conflict; BSD and CDDL software can mix.
I think ubuntu has a good argument[1] that it can still be shipped as a separate kernel module and satisfy both licenses.
"The CDDL applies to all files under the CDDL, while the GPLv2 applies to derivative works. The CDDL cannot apply to the Linux kernel because zfs.ko is a self-contained file system module — the kernel itself is quite obviously not a derivative work of this new file system."
Using the ZoL tree with FreeBSD doesn't have the same licensing conflict; BSD and CDDL software can mix.