It's not a GPL violation. OpenZFS works under Linux through a compatibility layer
called SPL, the Solaris Porting Layer. SPL is licensed under GPL. Torvalds himself said
that a non-GPL file system that was written for another OS cannot be considered a
derivative of the Linux kernel: https://yarchive.net/comp/linux/gpl_modules.html
SPL is a derived work from the Linux kernel because it's designed for the Linux
kernel. SPL is therefore under GPL. ZFS is designed for Solaris and therefore a different
license is fine.
Dell, a friggin huge US company, wouldn't distribute Ubuntu with their laptops if they
as the distributor did something illegal.