It seems perverse to say the least that freebsd was willing to incorporate the (more restrictive) CDDL[?]-licensed ZFS driver into their kernel, but won't include GPL code.
The CDDL is a file-scope weak copyleft license, and is (broadly) less restrictive than the GPL. As bunderbunder pointed out FreeBSD is not able to include GPL code, because the GPL is not compatible with the distribution terms for the kernel.
The CDDL is generally less restrictive in terms than the GPL; its not GPL-compatible, which, given the wide array of software under the GPL and the GPLs more intense restrictions make it hard to use in many places where GPL software is in use, which perhaps makes it seem restrictive in an environment where there is an unavoidable commitment to GPL software in some role which would create a requirement GPL- or GPL-compatible software, but that's a result of the of the fact that the GPL is restrictive, not the CDDL.
(The CDDL is decidedly not permissive, of course, but its a far cry from being more restrictive than the GPL.)
How is the CDDL more restrictive than the GPL ? As far as I can tell, ZFS can be included because the CDDL imposes less restrictions on the final distribution (the non-CDDL parts don't have to abide by the CDDL license, which would be the case with the GPL).
