Marvel claims the firms' are responsible because the game is played on servers operated by the companies, raising the question of whether a company is responsible for their customers' actions on its computer server. Marvel also claims the companies have disrupted its "existing and future" business prospects for licensing its characters in video games similar to City of Heroes.
It really can't be said enough: Fuck you, Marvel. Remember, these are the guys who teamed up with DC to copyright "superhero". Litigation happy bastards.
