AROS Cybergraphics is illegal?
As long as it is based on the P96 API, it is. There is likely a native CGfx API, though.
Has Frank Mariak claimed so?
I do not know what the conditions are for a third party to offer a CGfx driver. I can only tell you want the conditions for the P96 API are.
Currently, the drivers use the P96 API, and *that* is not valid.