Nvidia cares about the people reverse-engineering CUDA, who don't have anything to reverse-engineer unless they actually download the SDK. Of course, when you download the SDK it has an associated license.
It's also possible to do a clean-room reverse where one group tests the hardware and software and writes specifications and another group who has never seen the hardware or docs then does the implementation. This has been legally tested and protected going back to at least the early 1980s.
It is possible, but a) it is expensive as hell to get enough engineers whom you can prove in court have no exposure to the original software (most SWE's would get some exposure just naturally in college nowadays, leave alone at any sort of job) and b) CUDA is constantly changing and updating and so you need to have this expensive clean-room process going constantly or else you will fall behind.
The most famous case of clean-room reverse engineering is for the original BIOS chips back in the early 1980's, where the chips themselves couldn't change- they were hardware! It's going to be orders of magnitude more difficult to do that for software packages that change regularly.
I disagree on Cuda changing constantly. The hardware is stable once sold and new devices are usually backwards compatible by at least a version or two. The API is also locked down for already deployed versions, can't go pulling the rug from paying customers. However, new versions of both hardware and Cuda do introduce new things that'll need addressed. I don't think it's much of a moving target though.
