Ask HN: About John Carmack, Oculus lawsuit and copied code?

ELI9: why is John Carmack being accused of copying code while he was one of the most important programmers to have contributed to First-Person gaming and real-time 3D visualization on the PC platform? Why should he, among all programmers, be considered guilty of copying other people's code, when he was the first one to make several 3D engines open-source and 3D real-time visualization software is most likely to be derived from his work and algorithms?