Oracle sued over Google reimplementing code that was under the GPL (OpenJDK) under the Apache 2.0 license (Apache Harmony). Google recently switched to the GPL code because of it. In the case of ZoL, the code is derived from the original code and is under the original license. The idea that ZoL is somehow more at risk is pure FUD.
My understanding is that the risk is mostly the other way around: it infringes Linux' GPL license (see https://sfconservancy.org/blog/2016/feb/25/zfs-and-linux/). Would Oracle have grounds to sue as a Linux copyright holder? They have successfully made API copyrightable, so they clearly have competent lawyers.
There are a few people who think that. There are plenty of others who do not. Those in charge of reviewing licensing in Linux distributions that have adopted it are generally of the opinion that it is okay, especially those in charge of Sabayon and Ubuntu. Gentoo came to similar conclusions long before them (>4 years before Ubuntu) and ships it in binary form on the LiveDVD. At this point, there are at least a few dozen distributions that have adopted ZFS in some form or another.
I don't see how anyone could read that and come away thinking that ZFS-on-Linux isn't kosher. Given as simple as it is to compile zfs.ko in the Ubuntu live environment, I'd bet money that's Canonical's backup plan should a literal interpretation of the kernel's license become a legal reality.
And here's to hoping that ZFS becomes so widespread that Oracle themselves end up redistributing it in their version of Linux, like they did with DTrace.
The legal situation of Google's use of Java APIs was always a tiny bit murky, but it was super-clear-cut compared to the use of ZFS-on-Linux.