It's advertised as a "clean room" re-implementation. What part would be illegal?
At least grounds for suing and starting an extensive discovery process and possibly a costly injunction...
It was clean-room implemented purely from the API surface and by trial-and-error with open CUDA code.
Namely:
"4.1 License Scope. The SDK is licensed for you to develop applications only for use in systems with NVIDIA GPUs."