ZFS-FUSE existed before ZoL did. The reason they moved to their SPL-based solution is because FUSE didn't want to make the required changes to support high performance file systems.
SPL now handles the guarantee to separate ZFS's userland components from Linux's constant internal kernelspace API churn. ZFS, internally, doesn't really know what a Linux is beyond a few little bits here and there.
SPL now handles the guarantee to separate ZFS's userland components from Linux's constant internal kernelspace API churn. ZFS, internally, doesn't really know what a Linux is beyond a few little bits here and there.