Oracle was the original copyright holder before the OpenSolaris fiasco and Illumos was forked.
There still are a lot of lines in a lot of files that were written by Sun employees which are now owned by Oracle. So the same way a kernel developer has a potential standing to sue canonical over their potential GPL violation, Oracle has the same potential standing to sue canonical over their potential CDDL violation.
It's just that oracle is ever so much more likely to sue and actually win (see: the google api's are copyrightable case)
Oracle as the sole copyright holder of zfs might be because thy lose potential Solaris sales when zfs is distributed with Linux this way. At least they could argue that before some court