Well how much work can you expect a for-profit-company to do in order to release their software as open source? They choose a licence that worked for them, then and there because of reasons. This included the work on ZFS.

There are operating systems that have licences that permit the ZFS code in the kernel. What reasons did SUN have to specifically block the Linux kernel but not BSD / Darwin ? The choice of license has everything to do whit what license SUN was comfortable using and nothing to do with keeping GNU/Linux out of the party.

If the answer is that a lot of their competitors at the time were invested in GNU/Linux and thus could not use their code, that is a very short sighted move.