I think the key words in your post are "their partners."
If somebody loads Ubuntu on a system and ships it without Canonical's knowledge/participation, then Ubuntu can't be sued over this. They're not a party - they distributed GPLv3 software under the GPLv3, and it is the duty of anybody doing redistribution to comply with the license. In the same way you can't sue some contributor to Grub over something Canonical does with it.
However, if Canonical enters into some kind of agreement with the system vendor then I'd think that would change things legally. Now Canonical is complicit in the distribution in some way, especially if money starts changing hands.
If you violate the GPL that is none of my concern, but if I pay you to violate the GPL then legally I'm more entangled. Apple can't claim they don't distribute iOS because Foxconn does the image loading, and so on.