“I am not a lawyer.” This is my understanding of the topic.
Ubuntu itself is GPL (or some variant like LGPL or Apache Commons). The sample rootfs is distributed separately from the NVIDIA drivers which, when applied to sample rootfs, cause this to be known as L4T instead of just Ubuntu.
You could refer to Ubuntu docs on licensing, but what it comes down to is that anyone can distribute Ubuntu so long as you don’t claim it as your own and don’t charge for Ubuntu other than a reasonable packaging fee. Things you have created can typically have any license or fee you choose if it doesn’t directly change or derive from Ubuntu. NVIDIA’s hardware drivers are a good example…they are basically added on top of Ubuntu in a separate step and NVIDIA does not claim ownership of Ubuntu (and Ubuntu does not claim ownership of the Linux kernel…it just redistributes). Had NVIDIA taken the Nouveau video driver and modified it instead of creating their own original driver there would have been a need to follow Nouveau’s licensing as a derivative work.
You’ll notice that NVIDIA has contributed changes to the Linux kernel itself, and those changes became public and are distributed as free. The files which are not integrated with the kernel (the separate drivers which talk to the kernel but which are not part of the kernel) are owned and licensed and controlled by NVIDIA…the source code of those files are not provided to the public without a non-disclosure agreement.