Linux is not in the public domain. Linux is a cancer that attaches itself in an intellectual property sense to everything it touches. That's the way that the license works.
Bio: Samuel Langhorne Clemens, better known by his pen name Mark Twain, was an American author and humorist. He wrote The Adventures of Tom Sawyer and its sequel, Adventures of Huckleberry Finn, the latter often called "The Great American Novel".