How come? There is a CUDA C++ and CUDA C toolchains available under a MIT license, large part s of which are contributed by NVIDIA.

How can they sue you for using something that they give you with a license that says "we allow you to do whatever you want with it" ?

the MIT license doesn't have an express patent grant. If Nvidia has a patent on some technology used by the open source code, they could sue you for patent infringement if you use it in a way that displeases them. What they can't do is sue you for copyright infringement.

Google v Oracle is still unsettled.

Most other legal precedent was that it was fine to clone an API.

> Most other legal precedent was that it was fine to clone an API.

CUDA is more than an API. It is a technology under copyright and very likely patented too. Even the API itself contains multiple reference to "CUDA" in function calls and variable name.

None of that protects it from being cloned under previous 9th circuit precedent except maybe patents, but I'm not aware of any patents that'd protect another against CUDA implementation.