Because there's a nonzero risk the judge won't order Warhammer to pay the defendant's legal fees. Meaning that even if the author of the book "wins" in court,

Because there's a nonzero risk the judge won't order Warhammer to pay the defendant's legal fees. Meaning that even if the author of the book "wins" in court, he might still lose a lot of money. And if he doesn't have a corp or LLC set up to sell the book, or hasn't jumped through all the bureaucratic hoops involved correctly, there's a nonzero risk that the Warhammer people could get a multi-million dollar judgment on him and take his house, cars, kid's college fund and most of his future salary. (The fact that he says he's using the proceeds from the book to pay for his daughter's schooling actually implies he doesn't have one of these legal structures set up.) That's how the US legal system works, unfortunately. Disclaimer: I am not a lawyer and this is not legal advice.