How can they charge a product that's not their own?

OpenZFS is licensed under a free software license. There's no mechanism by which they could demand anything. At best they could try to sue Canonical for CDDL violations, though the legal arguments online suggest that Oracle wouldn't have a leg to stand on here because the license incompatibility comes from the GPL side (meaning that Linux copyright holders might be able to sue Canonical but even that is a stretch since it's hard to argue OpenZFS is a derivative work of Linux).

The thread you linked is someone who was using software with a proprietary license against its terms and is being asked to pay for its usage -- obviously I think Oracle is being scummy but it's not a comparable situation at all. This would be like saying that you won't use VS Code because Microsoft once demanded that someone who was using a cracked copy of Windows pay them -- it's a complete non-sequitur.

Would you refuse to use ZFS on FreeBSD as well?