FreeBSD is a first-class citizen in the OpenZFS project. It's not a hack or compatibility layer on top of the linux codebase.

It makes a great deal of sense for the Linux and FreeBSD communities to collaborate on ZFS rather than both maintaining their own separate forks and merging back and forth. It means you much more easily maintain feature & bugfix parity, and makes migrating filesystems between OSes easier.

It's a win for everyone involved, not a negative.