FreeBSD's license is GPL-compatible[1]. I believe they just prefer not to use GPL code.

[1] https://en.wikipedia.org/wiki/BSD_licenses#2-clause_license_...

When the FSF says a license is GPL-compatible, what they mean is, "You can legally combine software under this license with software under the GPL to produce a new combined product, provided that new product is distributed under the GPL."

Assuming that FreeBSD's maintainers want to keep their code under the BSD license, they'll be wanting something more permissive than the FSF's "heads I win tails you lose" version of compatibility.

Yes; but if they want to release the BSD kernel as a whole under the FreeBSD license, then they already cannot incorporate code that has any other license whatsoever. For example, they wouldn't accept my patch under the hypothetical license "you can do whatever you want with this code except shoot bunnies". If they did, FreeBSD users couldn't use FreeBSD to shoot bunnies.

Am I missing something about the BSD kernel licensing social dynamic?

It's not about patches. FreeBSD drivers generally shouldn't be patches; they should be loadable modules so you can use them without recompiling the kernel.

Now if you changed your hypothetical so that it's not a patch but a separate module that compiles to its own distinct binary, and your hypothetical license were expanded to include a second clause, "Anything that links to binaries built from this code must inherit this license," then we'd be talking about a situation that replicates the issues with making use of GPL code in projects that use more permissive licenses.

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).